Do NRVO when type of returned local does not match return type

[ecstatic-morse]

#72205 will fail to fire for code such as the following:

fn foo(x: &mut i32) -> &i32 { x }

This is because, by the time we get to the RenameReturnPlace pass, the reborrow has been optimized away and we have MIR like:

_0: &i32 = _1: &mut i32

To merge these locals, we need to pick a type for the unified LocalDecl. However, I wasn't sure how or if codegen makes use of the types of locals, so for now #72205 just doesn't run if the locals have different type. We should come up with a solution here.

[ecstatic-morse]

Worth noting that an initial perf run of #72205 was a bit faster than the one for the version that was merged. This issue is one plausible explanation, although there was also an LLVM upgrade in between.