Fix an anti-dep breaker corner case.

<rdar://problem/8959122> illegal register operands for UMULL instruction in cfrac nightly test
I'm stil working on a unit test, but the case is:
rx = movcc rx, r3
r2 = ldr
r2, r3 = umull r2, r2

The anti-dep breaker should not convert this into an illegal instruction:
r2, r2 = umull

llvm-svn: 124932