AMDGPU: Add Selection patterns to support add of one bit.

Summary:
  We generate s_xor to lower add of i1s in general cases, and s_not to
lower add with a one-bit imm of -1 (true).

Reviewers:
  rampitec

Differential Revision:
  https://reviews.llvm.org/D52518

llvm-svn: 343030