[PowerPC] Spill CR LT bits on P9 using setb

This patch aims to spill CR[0-7]LT bits on POWER9 using the setb instruction.
The sequence on P9 to spill these bits will be:

setb %reg, %CRREG
stw %reg, $FI

Instead of the typical sequence:

mfocrf %reg, %CRREG
rlwinm %reg1, %reg, $SH, 0, 0
stw %reg1, $FI

Differential Revision: https://reviews.llvm.org/D68443