[PowerPC] mapping hardward loop intrinsics to powerpc pseudo

Map hardware loop intrinsics loop_decrement and set_loop_iteration
to the new PowerPC pseudo instructions, so that the hardware loop
intrinsics will be expanded to normal cmp+branch form or ctrloop
form based on the CTR register usage on MIR level.

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D123366