[AArch64] Predicate SSHLL;SCVTF patterns behind UseAlternateSExtLoadCVTF32

There have been some patterns in the AArch64 backend to optimize code of
the form:
  ldrsh w8, [x0]
  scvtf s0, w8
to:
  ldr h0, [x0]
  sshll v0.4s, v0.4h, #0
  scvtf s0, s0
The idea is to remove the GRP->FPR move, but in reality is making code
larger and slower (or the same) on all the cpus I tried.

This patch adds the UseAlternateSExtLoadCVTF32 predicate similar to
nearby related pattern.

Differential Revision: https://reviews.llvm.org/D125470