[AArch64][SVE][CodeGen] Generate fused mul+add/sub ops with one of add/sub operands as splat

Currently, depending upon whether the add/sub instruction can synthesize immediate directly,
its decided whether to generate mul+(add/sub immediate) or mov+mla/mad/msb/mls ops.

If the add/sub can synthesize immediate directly, then fused ops wont get generated. This
patch tries to address this by having makeshift higher priority for the fused ops.

Specifically, patch aims at transformation similar to below:
add ( mul, splat_vector(C))
->
      MOV C
      MAD

Differential Revision: https://reviews.llvm.org/D142656