[MCA] Enable customization of individual instructions (#155420)

Currently MCA takes instruction properties from scheduling model.
However, some instructions may execute differently depending on external
factors - for example, latency of memory instructions may vary
differently depending on whether the load comes from L1 cache, L2 or
DRAM. While MCA as a static analysis tool cannot model such differences
(and currently takes some static decision, e.g. all memory ops are
treated as L1 accesses), it makes sense to allow manual modification of
instruction properties to model different behavior (e.g. sensitivity of
code performance to cache misses in particular load instruction). This
patch addresses this need.

The library modification is intentionally generic - arbitrary
modifications to InstrDesc are allowed. The tool support is currently
limited to changing instruction latencies (single number applies to all
output arguments and MaxLatency) via coments in the input assembler
code; the format is the like this:

add (%eax), eax // LLVM-MCA-LATENCY:100

Users of MCA library can already make additional customizations; command
line tool can be extended in the future.

Note that InstructionView currently shows per-instruction information
according to scheduling model and is not affected by this change.

See https://github.com/llvm/llvm-project/issues/133429 for additional
clarifications (including explanation why existing customization
mechanisms do not provide required functionality)

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Co-authored-by: Min-Yih Hsu <min@myhsu.dev>

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