Improve IR for calls with immediate return values

There are lots of unneeded allocas and Store/Load cycles for calls with
immediate return values. This is a first step towards removing that, allowing
immediate return values to be directly returned from trans_call_inner and
trans_lang_call (for now), instead of always stuffing them into an alloca.

For now, only a few things take advantage of the new behaviour, but this
already saves 16k allocas and 43k lines in total in the unoptimized IR
for librustc. Running "make check" under time shows that CPU time for
the unoptimized test suite is reduced by about 7%.