[ARM] Sink splats to MVE intrinsics

The predicated MVE intrinsics are generated as, for example,
llvm.arm.mve.add.predicated(x, splat(y). p). We need to sink the splat
value back into the loop, like we do for other instructions, so we can
re-select qr variants.

Differential Revision: https://reviews.llvm.org/D87693