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

[Nathan Gauër via llvm-commits]

================
@@ -422,6 +431,10 @@ class SPIRVGlobalRegistry : public SPIRVIRMapping {
   // structures referring this instruction.
   void invalidateMachineInstr(MachineInstr *MI);
 
+  // Return the FPVariant of to the given floating-point regiester.
----------------
Keenuts wrote:

```suggestion
  // Return the FPVariant of to the given floating-point register.
```

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