[InstCombine] canonicalize sign-bit-shift of difference to ext(icmp)

icmp is the preferred spelling in IR because icmp analysis is
expected to be better than any other analysis. This should
lead to more follow-on folding potential.

It's difficult to say exactly what we should do in codegen to
compensate. For example on AArch64, which of these is preferred:
	sub	w8, w0, w1
	lsr	w0, w8, #31

vs:
	cmp	w0, w1
	cset	w0, lt

If there are perf regressions, then we should deal with those in
codegen on a case-by-case basis.

A possible motivating example for better optimization is shown in:
https://llvm.org/PR43198 but that will require other transforms
before anything changes there.

Alive proof:
https://rise4fun.com/Alive/o4E

  Name: sign-bit splat
  Pre: C1 == (width(%x) - 1)
  %s = sub nsw %x, %y
  %r = ashr %s, C1
  =>
  %c = icmp slt %x, %y
  %r = sext %c

  Name: sign-bit LSB
  Pre: C1 == (width(%x) - 1)
  %s = sub nsw %x, %y
  %r = lshr %s, C1
  =>
  %c = icmp slt %x, %y
  %r = zext %c