[llvm] [RISCV] Update comment on -w stripping pass. NFC (PR #67415)

[Philip Reames via llvm-commits]

preames wrote:

A bit off topic, but should we be restricting this only to the current list?  I've been noticing a bunch of tests recently with RV32 vs RV64 differences on ADDIW vs ADDI where the difference is purely syntactic.  Maybe from a testing perspective alone, we should just strip all the legal w suffixes as long as it doesn't *decrease* compressability?  

https://github.com/llvm/llvm-project/pull/67415