[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

llvm-svn: 355130

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