[X86] Don't form extloads in combineExtInVec unless the load extension is legal.
[X86] Don't form extloads in combineExtInVec unless the load extension is legal.

This should prevent doing this on pre-sse4.1 targets or for 256
bit vectors without avx2.

I don't know of a failure from this. Op legalization will probably
take care of, but seemed better to be safe.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@365577 91177308-0d34-0410-b5e6-96231b3b80d8

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