[InstCombine][AMDGPU] Trim components of s_buffer_load

Summary:
Add trimming of unused components of s_buffer_load.

For s_buffer_load and unformatted buffer_load also trim unused
components at the beginning of vector and update offset accordingly.

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71785