[ZoneAlgo] Use getDefToTarget in makeValInst. NFC.

Move the optimized getDefToTarget() from ForwardOpTree to ZoneAlgo such
that it can be used by makeValInst.

This reduces the compile time of GrTestUtils of the aosp buildbot from
2m46s to 21s, which should fix the timeout issue.

Differential Revision: https://reviews.llvm.org/D48579

llvm-svn: 335606