Rather than having a ton of patterns for double shift instructions, e.g. SHLD16rrCL...

Rather than having a ton of patterns for double shift instructions, e.g. SHLD16rrCL, just perform custom dag combine to form x86 specific dag so they match to the same pattern. This also makes sure later dag combine do not cause isel to miss them (e.g. promoting i16 to i32).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102485 91177308-0d34-0410-b5e6-96231b3b80d8