[InstCombine] add tests for bswap disguised as shuffle; NFC
[InstCombine] add tests for bswap disguised as shuffle; NFC

Somewhat motivating case In PR43146:
https://bugs.llvm.org/show_bug.cgi?id=43146

But that's a lot more complicated.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@370381 91177308-0d34-0410-b5e6-96231b3b80d8

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