Update mlir-opt path resolution and opt/llc flags

[guillon]

Motivation

Fix unexpected behavior update flags in opt/llc.

Description

Fixes:

• mlir-opt: fix path search for mlir-opt in case of llvm/mlir local build
• codecov: make low patch coverage only informational (i.e. does not fail CI) as a patch coverage on previously low covered code may be very low

Changes:

• remove use of fast-math options which are too aggressive and incompatible with recent mlir opt versions
• change -O3 to -O2 in llc/opt in order to better preserve expected schedule

Discussions

Loop explore has been rerun on matmul (stratgey tile8dv) against tvm and previous version:

• no visible build time change
• no regression on top performance (~92% peak perf on an avx512 laptop) and overall search space distribution
• some expected regression on points where llc unrolled more and gained performance against tvm
• code size gain as opt/llc do not perform aggressive loop unrolling

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/xtc/utils/tools.py	0.00%	4 Missing ⚠️

📢 Thoughts on this report? Let us know!

[qaco]

Nice!