[InstCombine][AMDGPU] Trim more components of *buffer_load

Summary:
Add trimming of unused components of s_buffer_load.

Extend trimming of *buffer_load to also include
unused components at the beginning of vectors and update offset.

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70315