AMDGPU: Re-organize the outer loop of SILoadStoreOptimizer

Summary:
The entire algorithm operates per basic-block, so for cache locality
it should be better to re-optimize a basic-block immediately rather than
in a separate loop.

I don't have performance measurements.

Change-Id: I85106570bd623c4ff277faaa50ee43258e1ddcc5

Reviewers: arsenm, rampitec

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

Differential Revision: https://reviews.llvm.org/D40344

llvm-svn: 319156