[AArch64] Avoid using intermediate integer registers for copying between source and destination floating point registers

In post-isel code, there are cases where there were redundant copies from a source FPR to an intermediate GPR in order to copy to a destination FPR. In this patch, we identify these patterns in post-isel peephole optimization and replace them with a direct FPR-to-FPR copy.
One example for this will be the insertion of the scalar result of 'uaddlv' neon intrinsic function into a destination vector. During instruction selection phase, 'uaddlv' result is copied to a GPR, & a vector insert instruction is matched separately to copy the previous result to a destination SIMD&FP register.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D142594