[ARM NEON] Define vfms_f32 on ARM, and all vfms using vfma.

r259537 added vfma/vfms to armv7, but the builtin was only lowered
on the AArch64 side. Instead of supporting it on ARM, get rid of it.

The vfms builtin lowered to:
  %nb = fsub float -0.0, %b
  %r = @llvm.fma.f32(%a, %nb, %c)

Instead, define the operation in terms of vfma, and swap the
multiplicands. It now lowers to:
  %na = fsub float -0.0, %a
  %r = @llvm.fma.f32(%na, %b, %c)

This matches the instruction more closely, and lets current LLVM
generate the "natural" operand ordering:
  fmls.2s v0, v1, v2
instead of the crooked (but equivalent):
  fmls.2s v0, v2, v1
Except for theses changes, assembly is identical.

LLVM accepts both commutations, and the LLVM tests in:
  test/CodeGen/AArch64/arm64-fmadd.ll
  test/CodeGen/AArch64/fp-dp3.ll
  test/CodeGen/AArch64/neon-fma.ll
  test/CodeGen/ARM/fusedMAC.ll
already check either the new one only, or both.

Also verified against the test-suite unittests.

llvm-svn: 266807