[PowerPC] Do not assume operand of ADDI is an immediate

After pseudo-expansion, we may end up with ADDI (add immediate)
instructions where the operand is not an immediate but a relocation.
For such instructions, attempts to get the immediate result in
assertion failures for obvious reasons.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=45432
(cherry picked from commit a56d057d)