[RegisterCoalescing] Don't move COPY if it would interfere with another value

Summary:
RegisterCoalescer::removePartialRedundancy tries to hoist B = A from
BB0/BB2 to BB1:

  BB1:
       ...
  BB0/BB2:  ----
       B = A;   |
       ...      |
       A = B;   |
         |-------
         |

It does so if a number of conditions are fulfilled. However, it failed
to check if B was used by any of the terminators in BB1. Since we must
insert B = A before the terminators (since it's not a terminator itself),
this means that we could erroneously insert a new definition of B before a
use of it.

Reviewers: wmi, qcolombet

Reviewed By: wmi

Subscribers: MatzeB, llvm-commits, sdardis

Differential Revision: https://reviews.llvm.org/D44918

llvm-svn: 328689