[IR] Accept non-Instruction in BinaryOperator::CreateWithCopiedFlags() (NFC)

The underlying copyIRFlags() API accepts arbitrary values and can
work with flags on operators (i.e. instructions or constant
expressions). Remove the arbitrary limitation that the
CreateWithCopiedFlags() API imposes, so we can directly pass through
values matched by PatternMatch, which can be constant expressions.

The attached test case works fine now, but would crash with an
upcoming change to not produce and constant expressions.