[X86] Add a combine for back to back VSRAI instructions

Expansion of SIGN_EXTEND_INREG can create a VSRAI instruction. If there is already a VSRAI after it, we should combine them into a larger VSRAI

Differential Revision: https://reviews.llvm.org/D54959

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347784 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed