[NFCI][InstCombine] Move store merging from `visitStoreInst()` into...

[NFCI][InstCombine] Move store merging from `visitStoreInst()` into `visitUnconditionalBranchInst()`

Summary:
As @nikic is pointing out in https://bugs.llvm.org/show_bug.cgi?id=46680#c5,
InstCombine should not have forward instruction scans,
so let's move this transform into the proper place.

This is pretty much NFCI.

Reviewers: nikic, spatel

Reviewed By: nikic

Subscribers: hiraditya, llvm-commits, nikic

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83670