[llvm] [CodeGen] Speed up ReachingDefAnalysis (NFC) (PR #100913)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> How did peak memory consumption change?

According to heaptrack, the peak memory consumption goes up from 119.4MB to 124.4MB.

According to `/usr/bin/time/ -v`, the max RSS goes up from 286,436KB to 287,972KB.

I could reduce the numbers a little bit by reserving fewer elements for `Storage`.  The current patch sets aside enough storage for `NumBlockIDs * NumRegUnits` reaching definitions.  In 95% cases, we have no more than half of those (that is, `NumBlockIDs * NumRegUnits / 2` reaching definitions).


https://github.com/llvm/llvm-project/pull/100913