Isolate common wait_end logic
Elide unnecessary ret slot allocas

When there is only a single store to the ret slot that dominates the
load that gets the value for the "ret" instruction, we can elide the
ret slot and directly return the operand of the dominating store
instruction. This is the same thing that clang does, except for a
special case that doesn't seem to affect us.

Fixes #8238
auto merge of #8270 : dotdash/rust/ret_alloca_elim, r=pcwalton

When there is only a single store to the ret slot that dominates the
load that gets the value for the "ret" instruction, we can elide the
ret slot and directly return the operand of the dominating store
instruction. This is the same thing that clang does, except for a
special case that doesn't seem to affect us.

Fixes #8238
auto merge of #8327 : sstewartgallus/rust/factor_out_waitqueue, r=bblum

I'm a bit disappointed that I couldn't figure out how to factor out more of the code implementing `extra::sync` but I feel this is an okay start. Also I added some documentation explaining that `WaitQueue` isn't thread safe, and needs an exclusive lock.
 
@bblum
Merge remote-tracking branch 'remotes/origin/master' into cleanup-iterators