[llvm] [SPIRV] Add FPVariant tracking for floating-point registers in SPIR-V (PR #156871)

[Nathan Gauër via llvm-commits]

https://github.com/Keenuts commented:

No issues with this, but seconding the question about the reuse of the existing `VRegToTypeMap`. Is there a performance need to store the FPVariant in a map, or can getFPVariantForVreg do a lookup in the generic map and then do the `isBFloatTy`/`isFloatTy` every time?

https://github.com/llvm/llvm-project/pull/156871