RegisterCoalescer: Defer clearing implicit_def lanes

We can't go back and recover the lanes if it turns
out the implicit_def really can't be erased.

Assume all lanes are valid if an unresolved conflict
is encountered. There aren't any tests where this
seems to matter either way, but this seems like a
safer option.

Fixes bug 39602

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed