[PowerPC] Custom lower known CR bit spills
For known CRBit spills, CRSET/CRUNSET, it is more efficient to load and spill
the known value instead of extracting the bit.
eg. This sequence is currently used to spill a CRUNSET:
crclr 4*cr5+lt mfocrf r3,4 rlwinm r3,r3,20,0,0 stw r3,132(r1)
This patch custom lower it to:
li r3,0 stw r3,132(r1)
Differential Revision: https://reviews.llvm.org/D61754