[AMDGPU] Check for unneeded shift mask in shift PatFrags.

[AMDGPU] Check for unneeded shift mask in shift PatFrags.

The existing constrained shift PatFrags only dealt with masked shift
from OpenCL front-ends. This change copies the
X86DAGToDAGISel::isUnneededShiftMask() function to AMDGPU and uses it in
the shift PatFrag predicates.

Differential Revision: https://reviews.llvm.org/D113448