Refactor tracking of MOV coalescing.

Refactor tracking of MOV coalescing.

Reg<->Reg MOV coalescing status is now a part of the HRegUsage.
This allows register allocation to query it two times without incurring
a performance penalty. This in turn allows to better keep track of
vreg<->vreg MOV coalescing so that all vregs in the coalesce chain
get the effective |dead_before| of the last vreg.

A small performance improvement has been observed because this allows
to coalesce even spilled vregs (previously only assigned ones).