[RegisterCoalescer] Fix crash on early clobbered subreg operands.

The issue was with processing two subregs of the same reg are used in the same
instruction (e.g. inline asm): "def early-clobber" and other just "def".
Register coalescer ran in bad recursion if the early clobbered subreg is second
in the following sequence of COPYs.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D127136