AArch64: fix EXPENSIVE_CHECKS for arm64_32.
AArch64: fix EXPENSIVE_CHECKS for arm64_32.

For some reason I'd decided to mark the end-result of a GOT load as
dead. It's clearly not (necessarily).

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371883 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm