This CL updates the SPIR-V writer to generate the OpConstantTrue,
OpConstantFalse and OpConstant instructions.
Bug: tint:5
Change-Id: I660554c491e4eb569e3902fce0973fae3f27e6c0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/17820
Reviewed-by: David Neto <dneto@google.com>