[GlobalISel] Clear unreachable blocks' contents after selection.

If these blocks are unreachable, then we can discard all of the instructions.
However, keep the block around because it may have an address taken or the
block may have a stale reference from a PHI somewhere. Instead of finding
those PHIs and fixing them up, just leave the block empty.

Differential Revision: https://reviews.llvm.org/D111201