Prevent dead uses in register coalescer after rematerialization

The coalescer does not check if register uses are available
at the point of rematerialization. If it attempts to rematerialize
an instruction with such uses it can end up with use without a def.

LiveRangeEdit does such check during rematerialization, so just
call LiveRangeEdit::allUsesAvailableAt() to avoid the problem.

Differential Revision: https://reviews.llvm.org/D106396