Reapply "[utils][UpdateLLCTestChecks] Add MIR support to update_llc_test_checks.py." (#164965) (#166575)

This change enables update_llc_test_checks.py to automatically generate
MIR checks for RUN lines that use `-stop-before` or `-stop-after` flags
allowing tests to verify intermediate compilation stages (e.g., after
instruction selection but before peephole optimizations) alongside the
final assembly output. If `-debug-only` flag is present in the run line it's
considered as the main point of interest for testing and stop flags above
are ignored (that is no MIR checks are generated).

This resulted from the scenario, when I needed to test two instruction
matching patterns where the later pattern in the peepholer reverts the
earlier pattern in the instruction selector and distinguish it from the
case when the earlier pattern didn't worked at all.

Initially created by Claude Sonnet 4.5 it was improved later to handle
conflicts in MIR <-> ASM prefixes and formatting.

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