Insert the start instruction directly into the map before the uses. This
prevents improperly re-visting sgpr->vgpr phi inputs multiple times
which
would trigger a use after free.
I don't particularly trust the iteration scheme here. This is also
unnecessarily revisting transitive users of a phi or reg_sequence for
every
input operand, but I will address that separately.
Fixes#130646. I also believe it fixes#130119, although that test fails
less consistently for me.
dea5aa73fa