Add validaton for SPV_KHR_8bit_storage + convert to/from floats. (#1990)

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.