InferAddressSpaces: Convert tests to opaque pointers

InferAddressSpaces: Convert tests to opaque pointers

Had constantexprs be mangled by the opaquify script; had to update
those lines manually:
  NVPTX/bug31948.ll
  AMDGPU/old-pass-regressions.ll
  AMDGPU/old-pass-regressions-inseltpoison.ll
  AMDGPU/infer-address-space.ll

Required re-reunning update_test_checks:
  AMDGPU/redundant-addrspacecast.ll

In AMDGPU/insert-pos-assert.ll, bitcast_insert_pos_assert_2 deleted a
getelementptr of 0 which I'm guessing was relevant. Replaced with an
offset 1 GEP to ensure another addrspacecast is inserted.

AMDGPU/infer-getelementptr.ll had one case improve by introducing an
inbounds.