Add Region.block property for single-block Regions

[tobiasgrosser]

This shortens the code needed to access the block in a single-block region and also documents the expectation that there is just a single block in the region.

[codecov]

Codecov Report

Patch coverage: 81.25% and no project coverage change.

Comparison is base (f9e6cd4) 87.43% compared to head (f90e239) 87.44%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #743   +/-   ##
=======================================
  Coverage   87.43%   87.44%           
=======================================
  Files         108      108           
  Lines       15840    15844    +4     
  Branches     2379     2379           
=======================================
+ Hits        13850    13854    +4     
  Misses       1556     1556           
  Partials      434      434           

Impacted Files	Coverage Δ
xdsl/frontend/code_generation.py	0.00% <0.00%> (ø)
xdsl/transforms/lower_mpi.py	92.56% <ø> (ø)
xdsl/dialects/scf.py	90.19% <60.00%> (ø)
xdsl/dialects/builtin.py	82.25% <100.00%> (ø)
xdsl/ir.py	83.73% <100.00%> (+0.09%)	⬆️
.../transforms/experimental/ConvertStencilToLLMLIR.py	96.53% <100.00%> (ø)

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