[X86] Improve combineVectorShiftImm

Summary:
Fold (shift (shift X, C2), C1) -> (shift X, (C1 + C2)) for logical as
well as arithmetic shifts. This is needed to prevent regressions from
an upcoming funnel shift expansion change.

While we're here, fold (VSRAI -1, C) -> -1 too.

Reviewers: RKSimon, craig.topper

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77300