InstCombine: Gracefully handle more allocas in the wrong address space

Officially this is currently required to always use the datalayout's
alloca address space. This may change in the future, and it's cleaner
to propagate the existing alloca's addrspace anyway.

This is a triple fix. Initially the change in simplifyAllocaArraySize
would drop the address space, but produce output. Fixing this hit an
assertion in the cast combine.

This patch also makes the changes to handle this situation from
a33e1280 dead, so eliminate
it. InstCombine should not take it upon itself to introduce
addrspacecasts, and preserve the original address space instead.