[InstCombine] try to pull 'not' of select into compare operands

not (select ?, (cmp TPred, ?, ?), (cmp FPred, ?, ?) -->
     select ?, (cmp TPred', ?, ?), (cmp FPred', ?, ?)

If both sides of the select are cmps, we can remove an instruction.
The case where only side is a cmp is deferred to a possible
follow-on patch.

We have a more general 'isFreeToInvert' analysis, but I'm not seeing
a way to use that more widely without inducing infinite looping
(opposing transforms).
Here, we flip the compare predicates directly, so we should not have
any danger by creating extra intermediate 'not' ops.

Alive proofs:
https://rise4fun.com/Alive/jKa

Name: both select values are compares - invert predicates
  %tcmp = icmp sle i32 %x, %y
  %fcmp = icmp ugt i32 %z, %w
  %sel = select i1 %cond, i1 %tcmp, i1 %fcmp
  %not = xor i1 %sel, true
=>
  %tcmp_not = icmp sgt i32 %x, %y
  %fcmp_not = icmp ule i32 %z, %w
  %not = select i1 %cond, i1 %tcmp_not, i1 %fcmp_not

Name: false val is compare - invert/not
  %fcmp = icmp ugt i32 %z, %w
  %sel = select i1 %cond, i1 %tcmp, i1 %fcmp
  %not = xor i1 %sel, true
=>
  %tcmp_not = xor i1 %tcmp, -1
  %fcmp_not = icmp ule i32 %z, %w
  %not = select i1 %cond, i1 %tcmp_not, i1 %fcmp_not

Differential Revision: https://reviews.llvm.org/D72007