[X86] scaleShuffleMask - avoid potential signed overflow warning.
[X86] scaleShuffleMask - avoid potential signed overflow warning.

Use size_t assignment to prevent a bad explicit type conversion warning.

Given the typical size of shuffle masks this was never going to happen, but this at least stops the warning.

Reported in https://www.viva64.com/en/b/0629/

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@359479 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm