[AMDGPU] Handle CreateBinOp not returning BinaryOperator (#137791)
AMDGPUCodeGenPrepareImpl::visitBinaryOperator() calls
Builder.CreateBinOp() and casts the resulting Value as a BinaryOperator
without checking, leading to an assert failure in a case found by
fuzzing. In this case, the operands are constant and CreateBinOp does
constant folding so returns a Constant instead of a BinaryOperator.