[SROA][Mem2Reg] Do not crash on alloca + addrspacecast

SROA knows that it can look through addrspacecast but
PromoteMemoryToRegister did not handle them. This caused an assertion
error for the test case, exposed while running
`Transforms/PhaseOrdering/inlining-alignment-assumptions.ll` with D83978
applied.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D84085