[x86] scalarize extract element 0 of FP math

[x86] scalarize extract element 0 of FP math

This is another step towards ensuring that we produce the optimal code for reductions,
but there are other potential benefits as seen in the tests diffs:

  1. Memory loads may get scalarized resulting in more efficient code.
  2. Memory stores may get scalarized resulting in more efficient code.
  3. Complex ops like fdiv/sqrt get scalarized which may be faster instructions depending on uarch.
  4. Even simple ops like addss/subss/mulss/roundss may result in faster operation/less frequency throttling when scalarized depending on uarch.

The TODO comment suggests 1 or more follow-ups for opcodes that can currently result in regressions.

Differential Revision: https://reviews.llvm.org/D58282

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355130 91177308-0d34-0410-b5e6-96231b3b80d8