[SPIRV] Support G_IS_FPCLASS (#148637)
This commit adds custom legalization for G_IS_FPCLASS, corresponding to
the @llvm.is.fpclass intrinsic.
The lowering strategy is essentially copied and adjusted from the
target-agnostic LegalizeHelper::lowerISFPCLASS legalization. The reason
we can't just use that directly is that the series of instruction it
expands to aren't logged in the SPIR-V backend's register/type
book-keeping, leading to issues later on in the compilation process.
As such the code introduced here was copied from the aforementioned
helper method, with some notable changes:
* Each new instruction's destination register must have a SPIR-V type
registered to it.
* Instead of a COPY from the floating-point type to integer, we issue a
SPIR-V OpBitcast directly. The backend doesn't currently appear to
handle bitcast-like COPYs.
Fixes #72862
