CodeGen: Fix TargetLowering::LowerCallTo for sret value type

TargetLowering::LowerCallTo assumes that sret value type corresponds to a
pointer in default address space, which is incorrect, since sret value type
should correspond to a pointer in alloca address space, which may not
be the default address space. This causes assertion for amdgcn target
in amdgiz environment.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D39996

llvm-svn: 318167