Also combine zext/sext into selects for ARM.

This turns common i1 patterns into predicated instructions:

  (add (zext cc), x) -> (select cc (add x, 1), x)
  (add (sext cc), x) -> (select cc (add x, -1), x)

For a function like:

  unsigned f(unsigned s, int x) {
    return s + (x>0);
  }

We now produce:

  cmp r1, #0
  it  gt
  addgt.w r0, r0, #1

Instead of:

  movs  r2, #0
  cmp r1, #0
  it  gt
  movgt r2, #1
  add r0, r2

llvm-svn: 162177