[PowerPC] Fix crash in peephole optimization

When converting reg+reg shifts to reg+imm rotates, we neglect to consider the
CodeGenOnly versions of the 32-bit shift mnemonics. This means we produce a
rotate with missing operands which causes a crash.

Committing this fix without review since it is non-controversial that the list
of mnemonics to consider should include the 64-bit aliases for the exact
mnemonics.

Fixes PR44183.