[GlobalISel] Fix CSEMIRBuilder silently allowing use-before-def.

[GlobalISel] Fix CSEMIRBuilder silently allowing use-before-def.

If a CSEMIRBuilder query hits the instruction at the current insert point,
move insert point ahead one so that subsequent uses of the builder don't end up with
uses before defs.

This fix also shows that AMDGPU was also affected by this bug often, but got away
with it because it was using a G_IMPLICIT_DEF before the use.

Differential Revision: https://reviews.llvm.org/D88605