Revert "Revert "[GlobalISel][IRTranslator] Emit trap intrinsic for "unreachable"""

This reverts commit d95cd811.

The selector sometimes leaves unreachable blocks unselected because it uses a
postorder traversal for the block ordering.

With the trap intrinsics now being emitted, these blocks are no longer empty and
the unselected G_INTRINSIC instructions survive past selection. To fix this,
keep track of which blocks are selected and later delete any blocks that weren't
selected.