Define patterns for shld and shrd that match immediate

Define patterns for shld and shrd that match immediate
shift counts, and patterns that match dynamic shift counts
when the subtract is obscured by a truncate node.

Add DAGCombiner support for recognizing rotate patterns
when the shift counts are defined by truncate nodes.

Fix and simplify the code for commuting shld and shrd
instructions to work even when the given instruction doesn't
have a parent, and when the caller needs a new instruction.

These changes allow LLVM to use the shld, shrd, rol, and ror
instructions on x86 to replace equivalent code using two
shifts and an or in many more cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57662 91177308-0d34-0410-b5e6-96231b3b80d8