[x86] use psubus for more vsetcc lowering (PR39859)

[x86] use psubus for more vsetcc lowering (PR39859)

Circling back to a leftover bit from PR39859:
https://bugs.llvm.org/show_bug.cgi?id=39859#c1

...we have this counter-intuitive (based on the test diffs) opportunity to use 'psubus'.
This appears to be the better perf option for both Haswell and Jaguar based on llvm-mca.
We already do this transform for the SETULT predicate, so this makes the code more
symmetrical too. If we have pminub/pminuw, we prefer those, so this should not affect
anything but pre-SSE4.1 subtargets.

  $ cat before.s
	movdqa	-16(%rip), %xmm2    ## xmm2 = [32768,32768,32768,32768,32768,32768,32768,32768]
	pxor	%xmm0, %xmm2
	pcmpgtw	-32(%rip), %xmm2 ## xmm2 = [255,255,255,255,255,255,255,255]
	pand	%xmm2, %xmm0
	pandn	%xmm1, %xmm2
	por	%xmm2, %xmm0

  $ cat after.s
	movdqa	-16(%rip), %xmm2    ## xmm2 = [256,256,256,256,256,256,256,256]
	psubusw	%xmm0, %xmm2
	pxor	%xmm3, %xmm3
	pcmpeqw	%xmm2, %xmm3
	pand	%xmm3, %xmm0
	pandn	%xmm1, %xmm3
	por	%xmm3, %xmm0

  $ llvm-mca before.s -mcpu=haswell
  Iterations:        100
  Instructions:      600
  Total Cycles:      909
  Total uOps:        700

  Dispatch Width:    4
  uOps Per Cycle:    0.77
  IPC:               0.66
  Block RThroughput: 1.8

  $ llvm-mca after.s -mcpu=haswell
  Iterations:        100
  Instructions:      700
  Total Cycles:      409
  Total uOps:        700

  Dispatch Width:    4
  uOps Per Cycle:    1.71
  IPC:               1.71
  Block RThroughput: 1.8

Differential Revision: https://reviews.llvm.org/D60838

llvm-svn: 358999