[clang] [llvm] [PowerPC] Implement 32-bit expansion for rldimi (PR #86783)

[Kai Luo via cfe-commits]

bzEq wrote:

> due to backward compatibility, it needs to be expanded into series of rlwimi in 32-bit environment

Why must be 'series of rlwimi'?

Why don't we just expand it following what ISA describes and let legalizer generates code sequence under 32-bit mode?
```
n ← sh5 || sh0:4
r ← ROTL64((RS), n)
b ← mb5 || mb0:4
m ← MASK(b, ¬n) RA ← r&m | (RA) & ¬m
```

https://github.com/llvm/llvm-project/pull/86783