MachineSink: Fix sinking VGPR def out of a divergent loop

This fixes sinking a VGPR def out of a loop past the reconvergence
point at the SI_END_CF. There was a prior fix which introduced
blockPrologueInterferes (D121277) to fix the same basic problem for
the post RA sink. This also had the special case isIgnorableUse case
which was incorrect, because in some contexts the exec use is not
ignorable.

I'm thinking about a new way to represent this which will avoid
needing hasIgnorableUse and isBasicBlockPrologue, which would function
more like the exception handling.

Fixes: SWDEV-407790

https://reviews.llvm.org/D155343