[InstCombine] add tests for ext-of-bool + add/sub; NFC

We should choose one of these as canonical:

  %z = zext i1 %cmp to i32
  %r = sub i32 %x, %z
  =>
  %s = sext i1 %cmp to i32
  %r = add i32 %x, %s

The test comments assume that the zext form is better,
but we can adjust that if we decide to go the other way.

llvm-svn: 352515