[IR] Remove the possibility of ConstantExpr having fast-math flags. (#94507)

This possibility was added in https://reviews.llvm.org/D34303 to resolve
some assertion failures with cases where FP math operations got
constant-folded in constant expressions. However, at no point did the IR
representation allow for expressing fast-math flags on constant
expressions.

With the change of #93038,
there are no longer any constant expressions capable of being FP math
operators, and thus FPMathOperator can go back to being
Instruction-only.

Author: jcranmer-intel

llvm/include/llvm/IR/Operator.h

@@ -330,8 +330,6 @@ class FPMathOperator : public Operator {
     unsigned Opcode;
     if (auto *I = dyn_cast<Instruction>(V))
       Opcode = I->getOpcode();
-    else if (auto *CE = dyn_cast<ConstantExpr>(V))
-      Opcode = CE->getOpcode();
     else
       return false;