[GlobalISel] Fix load-or combine moving loads across potential aliasing stores.

Although this combine checks that there's no load folding barriers between
the loads that it's trying to merge, it was inserting the load at the
MIRBuilder's default insertion point, which is the G_OR use inst.

This was causing a miscompile in the test suite's
SingleSource/Regression/C/gcc-c-torture/execute/GCC-C-execute-bswap-2

Differential Revision: https://reviews.llvm.org/D106251