[AMDGPU] SILoadStoreOptimizer: Allow merging across a swizzled access

Swizzled accesses are not merged, but there is no particular reason not
to merge two instructions if any of the intervening instructions happens
to be a swizzled access.

This moves the check for swizzled accesses out of checkAndPrepareMerge
into collectMergeableInsts where I think it makes more sense.

Differential Revision: https://reviews.llvm.org/D118267