[PowerPC] Split s34imm into two types

Currently the instruction paddi always takes s34imm as the type for the
34 bit immediate. However, the PC Relative form of the instruction should
not produce the same fixup as the non PC Relative form.
This patch splits the s34imm type into s34imm and s34imm_pcrel so that two
different fixups can be emitted.

Reviewed By: nemanjai, #powerpc, kamaub

Differential Revision: https://reviews.llvm.org/D83255