CodeGen: Fix insertion position of addrspace cast for alloca

For non-zero alloca addr space, alloca is usually casted to default addr
space immediately.

For non-vla, alloca is inserted at AllocaInsertPt, therefore the addr
space cast should also be insterted at AllocaInsertPt. However,
for vla, alloca is inserted at the current insertion point of IRBuilder,
therefore the addr space cast should also inserted at the current
insertion point of IRBuilder.

Currently clang always insert addr space cast at AllocaInsertPt, which
causes invalid IR.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D39374

llvm-svn: 316909