GlobalISel: Try to fold G_SEXT_INREG to G_AND with mask

GlobalISel: Try to fold G_SEXT_INREG to G_AND with mask

Copies the same transform from the DAG. Helps eliminate some codegen
diffs to allowed shared checks in a future change.

Not sure if apply supports anything better than C++ fragments for
the result. It's also not really reasonable that every combine has
to set the default insertion point.