[PowerPC] PPC backend optimization to lower int_ppc_tdw/int_ppc_tw intrinsics...

[PowerPC] PPC backend optimization to lower int_ppc_tdw/int_ppc_tw intrinsics to TDI/TWI machine instructions

This patch adds the backend optimization to match XL behavior for the two
builtins __tdw and __tw that when the second input argument is an immediate,
emitting tdi/twi instructions instead of td/tw.

Reviewed By: nemanjai, amyk, PowerPC

Differential revision: https://reviews.llvm.org/D112285