[PowerPC] Fix the check for 16-bit signed field in peephole

When a D-Form instruction is fed by an add-immediate, we attempt
to merge the two immediates to form a single displacement so we
can remove the add-immediate.

However, we don't check whether the new displacement fits into
a 16-bit signed immediate field early enough. Namely, we do a
sign-extend from 16 bits first which will discard high bits and
then we check whether the result is a 16-bit signed immediate.
It of course will always be.

Move the check prior to the sign extend to ensure we are checking
the correct value.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49640