[RegisterCoalescer] Make resolveConflicts aware of earlyclobber

Prior to this patch, it skipped the instruction defining VNI when checking if the tainted lanes are used.
In the given example, VRGATHER is an illegal instruction because its DstReg overlaps with SrcReg.

Therefore we need to check the defining instruction as well when there is an earlyclobber constraint.

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D105684