[GlobalISel] Fix multiply with overflow intrinsics legalization generating invalid MIR. During lowering of G_UMULO and friends, the previous code moved the builder's insertion point to be after the legalizing instruction. When that happened, if there happened to be a "G_CONSTANT i32 0" immediately after, the CSEMIRBuilder would try to find that constant during the buildConstant(zero) call, and since it dominates itself would return the iterator unchanged, even though the def of the constant was *after* the current insertion point. This resulted in the compare being generated *before* the constant which it was using. There's no need to modify the insertion point before building the mul-hi or constant. Delaying moving the insert point ensures those are built/CSEd before the G_ICMP is built. Fixes PR47679 Differential Revision: https://reviews.llvm.org/D88514 (cherry picked from commit 1d54e75cf26a4c60b66659d5d9c62f4bb9452b03)