[AArch64] Generate FADDP from shuffled fadd

[AArch64] Generate FADDP from shuffled fadd

As a follow up to D126686, this does the same fold for floating point
add and shuffle. In this case it is limited to reassoc either x[0]+x[1]
or x[1]+x[0] for both result[0] and results[1].

Differential Revision: https://reviews.llvm.org/D127087