[ARM] CSEL generation
This adds a peephole optimisation to turn a t2MOVccr that could not be
folded into any other instruction into a CSEL on 8.1-m. The t2MOVccr
would usually be expanded into a conditional mov, that becomes an IT;
MOV pair. We can instead generate a CSEL instruction, which can
potentially be smaller and allows better register allocation freedom,
which can help reduce codesize. Performance is more variable and may
depend on the micrarchitecture details, but initial results look good.
If we need to control this per-cpu, we can add a subtarget feature as we
Original patch by David Penry.
Differential Revision: https://reviews.llvm.org/D83566