[PowerPC][Peephole] Check if `extsw`'s second operand is a virtual register

Summary:
When combining `extsw` and `sldi` in `PPCMIPeephole`, we have to check
if `extsw`'s second operand is a virtual register, otherwise we might
get miscompile.

Differential Revision: https://reviews.llvm.org/D65315

llvm-svn: 367645