[llvm] [AMDGPU] Use FPImmLeaf for float constants in tablegen. NFCI. (PR #178018)

[Matt Arsenault via llvm-commits]

================
@@ -746,23 +746,9 @@ int FP64_NEG_ONE = 0xbff0000000000000;
 }
 def CONST : Constants;
 
-def FP_ZERO : PatLeaf <
-  (fpimm),
-  [{return N->getValueAPF().isZero();}]
->;
-
-def FP_ONE : PatLeaf <
-  (fpimm),
-  [{return N->isExactlyValue(1.0);}]
->;
-
-def FP_HALF : PatLeaf <
-  (fpimm),
-  [{return N->isExactlyValue(0.5);}]> {
-  let GISelPredicateCode = [{
-    return MI.getOperand(1).getFPImm()->isExactlyValue(0.5);
-  }];
-}
+def fpimm_zero : FPImmLeaf<fAny, [{ return Imm.isZero(); }]> ;
----------------
arsenm wrote:

```suggestion
def fpimm_zero : FPImmLeaf<fAny, [{ return Imm.isZero() && !Imm.isNegative(); }]> ;
```

https://github.com/llvm/llvm-project/pull/178018