SILGen: Fix assertion failure when accessing a property with a __consuming getter
We checked isMutating() on the accessor, and if it was false, called
isNonMutatingSelfIndirect(), which would assert that the accessor
satisfies isNonMutating().
However, isNonMutating() is not the opposite of isMutating(); rather,
one of isMutating(), isNonMutating() or isConsuming() is true.
Change the assertion to reflect this case.
Fixes rdar://problem/74095841.