Recommit "[DSE] Track earliest escape, use for loads in isReadClobber."

This reverts the revert commit df56fc6e.

This version of the patch adjusts the location where the EarliestEscapes
cache is cleared when an instruction gets removed. The earliest escaping
instruction does not have to be a memory instruction.

It could be a ptrtoint instruction like in the added test
@earliest_escape_ptrtoint, which subsequently gets removed. We need to
invalidate the EarliestEscape entry referring to the ptrtoint when
deleting it.

This fixes the crash mentioned in
https://bugs.chromium.org/p/chromium/issues/detail?id=1252762#c6