[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

llvm-svn: 360677