[SystemZ] implement shouldCoalesce()

Implement shouldCoalesce() to help regalloc avoid running out of GR128
registers.

If a COPY involving a subreg of a GR128 is coalesced, the live range of the
GR128 virtual register will be extended. If this happens where there are
enough phys-reg clobbers present, regalloc will run out of registers (if
there is not a single GR128 allocatable register available).

This patch tries to allow coalescing only when it can prove that this will be
safe by checking the (local) interval in question.

Review: Ulrich Weigand, Quentin Colombet
https://reviews.llvm.org/D37899
https://bugs.llvm.org/show_bug.cgi?id=34610

llvm-svn: 314516