[X86][SSE] Add support for detecting SUB(SPLAT_BV, SPLAT) cases for shift-rotate patterns.

This improves splat rotations (rotation by an uniform value), to avoid having to use the generic non-uniform shift code (extension to PR37426).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333641 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed