[X86] Limit getTargetConstantFromNode to only work on NormalLoads not extending loads.
[X86] Limit getTargetConstantFromNode to only work on NormalLoads not extending loads.

This seems to fix a failure reported by Jordan Rupprecht, but we
don't have a reduced test case yet.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@365589 91177308-0d34-0410-b5e6-96231b3b80d8

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