Fix some issues with rematerialization in RegisterCoalescer when the destination
of the copy is a subregister def. The current code assumes that it can do a full
def of the destination register, but it is not checking that the def operand is
read-undef. It also doesn't clear the subregister index of the destination in
the new instruction to reflect the full subregister def.

These issues were found running 'make check' with my next commit that enables
rematerialization in more cases.
Fix RegisterCoalescer::rematerializeTrivialDef() so that it works on flipped
CoalescerPairs. Also, make it take a CoalescerPair directly like other methods
of RegisterCoalescer.