[AMDGPU] SILoadStoreOptimizer: Remove redundant check for volatile

SILoadStoreOptimizer::collectMergeableInsts already ends the current
block if it sees a volatile (or ordered) memory access, so there is no
need to check for them again when scanning the instructions between two
pairing candidates in a block.

Differential Revision: https://reviews.llvm.org/D118266