[DSE] Fall back to CFG scan for unreachable terminators.

Blocks with UnreachableInst terminators are considered as root nodes in
the PDT. This pessimize DSE, if there are no aliasing reads from the
potentially dead store and the block with the unreachable terminator.

If any of the root nodes of the PDF has UnreachableInst as terminator,
fall back to the CFG scan, even the common dominator of all killing
blocks does not post-dominate the block with potentially dead store.

It looks like the compile-time impact for the extra scans is negligible.
https://llvm-compile-time-tracker.com/compare.php?from=779bbbf27fe631154bdfaac7a443f198d4654688&to=ac59945f1bec1c6a7d7f5590c8c69fd9c5369c53&stat=instructions

Fixes #53800.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D119760