[ARM] Sink splats to vector float instructions

Some MVE floating point instructions have gpr register variants that take
the scalar gpr value and splat them to all lanes. In order to accept
them in loops, the shuffle_vector and insert need to be sunk down into
the loop, next to the instruction so that ISel can see the whole
pattern.

This does that sinking for FAdd, FSub, FMul and FCmp. The patterns for
mul are slightly more constrained as there are no fms variants taking
register arguments.

Differential Revision: https://reviews.llvm.org/D76023