Fix for pMustExpand early fail.

Fix for pMustExpand early fail.

When EE tells Jit to enforce intrinsic expansion, we also checked whether
the intrinsic will be turned into a user call by IsIntrinsicImplementedByUserCall.
In fact, this API is primarily for math intrinsic and thus somewhat ambiguous since
even for any arbitrary non-math intrinsic ID, it returns true.
There are some side-effects, so I didn't refactor this API in this change.
Instead, the fix for pMustExpand is to check whether gtFlags has GTF_CALL.