The SPV_KHR_8bit_storage extension does not permit 8-bit integers to be
cast directly to floating point types. We are seeing shaders in the
wild, being produced by toolchains like glslang, that are generating
invalid SPIR-V.
This change adds validation to check for the patterns not permitted, and
some tests that expose the failure.
This CL moves the various validate files into the val/ directory with
the rest of the validation infrastructure. This matches how opt/ is
setup with the passes with the infrastructure.
