[async] Classify tasks to make fusion 10.6x faster

[xumingkuan]

Related issue = #742

Benchmark: async_mgpcg.py, taichi::lang::StateFlowGraph::fuse:
Before: 2.815 s
At commit 691f706: 520.011 ms
At commit 3c78343: 332.199 ms
At commit 42da16c: 283.921 ms
At commit 366af3d (buggy): 197.838 ms
At commit 826d053: 241.768 ms
After (at commit 6e6892f): 152.634 ms

On kun: ~1.03s -> ~97ms (10.6x faster)

The following statistics may change even at the same commit...
Without fusion:

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4224.00
codegen_offloaded_tasks: 117.00
codegen_statements  : 4323.00
launched_tasks      : 5034.00
launched_tasks_compute: 4990.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 106.00
launched_tasks_struct_for: 4883.00

With fusion (before this PR):

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4529.00
codegen_offloaded_tasks: 101.00
codegen_statements  : 4628.00
launched_tasks      : 4891.00
launched_tasks_compute: 4847.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 37.00
launched_tasks_struct_for: 4809.00

With fusion (at commit 691f706):

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4493.00
codegen_offloaded_tasks: 98.00
codegen_statements  : 4581.00
launched_tasks      : 4889.00
launched_tasks_compute: 4845.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 35.00
launched_tasks_struct_for: 4809.00

With fusion (at commit 42da16c):

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4399.00
codegen_offloaded_tasks: 94.00
codegen_statements  : 4498.00
launched_tasks      : 4880.00
launched_tasks_compute: 4836.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 26.00
launched_tasks_struct_for: 4809.00

With fusion (at commit 366af3d):

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4397.00
codegen_offloaded_tasks: 93.00
codegen_statements  : 4496.00
launched_tasks      : 4879.00
launched_tasks_compute: 4835.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 25.00
launched_tasks_struct_for: 4809.00

With fusion (at commit 826d053):

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4367.00
codegen_offloaded_tasks: 92.00
codegen_statements  : 4466.00
launched_tasks      : 4880.00
launched_tasks_compute: 4836.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 26.00
launched_tasks_struct_for: 4809.00

With fusion (at commit 6e6892f):

codegen_accessor_statements: 20.00
codegen_evaluator_statements: 79.00
codegen_kernel_statements: 4395.00
codegen_offloaded_tasks: 94.00
codegen_statements  : 4494.00
launched_tasks      : 4880.00
launched_tasks_compute: 4836.00
launched_tasks_list_gen: 44.00
launched_tasks_list_op: 44.00
launched_tasks_range_for: 1.00
launched_tasks_serial: 26.00
launched_tasks_struct_for: 4809.00

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---

[codecov]

Codecov Report

Merging #1887 into master will increase coverage by 0.61%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1887      +/-   ##
==========================================
+ Coverage   43.19%   43.80%   +0.61%     
==========================================
  Files          44       45       +1     
  Lines        6330     6168     -162     
  Branches     1092     1097       +5     
==========================================
- Hits         2734     2702      -32     
+ Misses       3426     3297     -129     
+ Partials      170      169       -1     

Impacted Files	Coverage Δ
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/testing.py	75.00% <0.00%> (-0.72%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/lang/meta.py	62.31% <0.00%> (-0.54%)	⬇️
python/taichi/misc/util.py	17.48% <0.00%> (-0.26%)	⬇️
python/taichi/misc/task.py	0.00% <0.00%> (ø)
python/taichi/cc_compose.py	0.00% <0.00%> (ø)
python/taichi/lang/shell.py	0.00% <0.00%> (ø)
python/taichi/core/__init__.py	0.00% <0.00%> (ø)
python/taichi/tools/patterns.py	0.00% <0.00%> (ø)
... and 13 more

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[yuanming-hu]

LGTM. Thank you!