[x86] allow movmsk with 2-element reductions

[x86] allow movmsk with 2-element reductions

One motivation for making this change is that the lack of using movmsk is likely
a main source of perf difference between clang and gcc on the C-Ray benchmark as
shown here:
https://www.phoronix.com/scan.php?page=article&item=gcc-clang-2019&num=5
...but this change alone isn't enough to solve that problem.

The 'all-of' examples show what is likely the worst case trade-off: we end up with
an extra instruction (or 2 if we count the 'xor' register clearing). The 'any-of'
examples look clearly better using movmsk because we've traded 2 vector instructions
for 2 scalar instructions, and movmsk may have better timing than the generic 'movq'.

If we examine the llvm-mca output for these cases, it appears that even though the
'all-of' movmsk variant looks worse on paper, it would perform better on both
Haswell and Jaguar.

  $ llvm-mca -mcpu=haswell no_movmsk.s -timeline
  Iterations:        100
  Instructions:      400
  Total Cycles:      504
  Total uOps:        400

  Dispatch Width:    4
  uOps Per Cycle:    0.79
  IPC:               0.79
  Block RThroughput: 1.0

  $ llvm-mca -mcpu=haswell movmsk.s -timeline
  Iterations:        100
  Instructions:      600
  Total Cycles:      358
  Total uOps:        600

  Dispatch Width:    4
  uOps Per Cycle:    1.68
  IPC:               1.68
  Block RThroughput: 1.5

  $ llvm-mca -mcpu=btver2 no_movmsk.s -timeline
  Iterations:        100
  Instructions:      400
  Total Cycles:      407
  Total uOps:        400

  Dispatch Width:    2
  uOps Per Cycle:    0.98
  IPC:               0.98
  Block RThroughput: 2.0

  $ llvm-mca -mcpu=btver2 movmsk.s -timeline
  Iterations:        100
  Instructions:      600
  Total Cycles:      311
  Total uOps:        600

  Dispatch Width:    2
  uOps Per Cycle:    1.93
  IPC:               1.93
  Block RThroughput: 3.0

Finally, there may be CPUs where movmsk is horribly slow (old AMD small cores?), but if
that's true, then we're also almost certainly making the wrong transform already for
reductions with >2 elements, so that should be fixed independently.

Differential Revision: https://reviews.llvm.org/D59997

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357367 91177308-0d34-0410-b5e6-96231b3b80d8