[AMDGPU][CodeGen] To improve CGEMM performance: combine LDS reads.

hange explores the fact that LDS reads may be reordered even if access
the same location.

Prior the change, algorithm immediately stops as soon as any memory
access encountered between loads that are expected to be merged
together. Although, Read-After-Read conflict cannot affect execution
correctness.

Improves hcBLAS CGEMM manually loop-unrolled kernels performance by 44%.
Also improvement expected on any massive sequences of reads from LDS.

Differential Revision: https://reviews.llvm.org/D25944

llvm-svn: 285919