This helps in the case of field projections of the results of checked
binary operations. E.g.:
_1 = CheckedAdd(const 1i32, const 2i32);
assert(!(_1.1: bool), "attempt to add with overflow" -> bb1
Previously, the `_1.1` field projection lvalue would force_allocate `_1`
so it could read the memory in the old-style way. Now checked math with
its assertions will not allocate at all.
The oom2.rs compile-fail test had to be re-written, because the old
version of it no longer allocates _at all_ (yay!), so it would hit the
stack depth limit instead, from recursion.
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