CodeGen: Insert addr space cast for automatic/temp var at right position

The uses of alloca may be in different blocks other than the block containing the alloca.
Therefore if the alloca addr space is non-zero and it needs to be casted to default
address space, the cast needs to be inserted in the same BB as the alloca insted of
the current builder insert point since the current insert point may be in a different BB.

Differential Revision: https://reviews.llvm.org/D35438

llvm-svn: 308313