Look at phis in ReplaceUsesDominatedBy.

When the instruction does not dominate the phi, but the instruction
that it is replacing does by being an input, do the replacement.

It was a missed improvement opportunity before, but the String.<init>
handling in the builder needs it for correctness.

bug: 111758226
Test: 563-checker-fakestring
Change-Id: I37f01e1aabc2a1f60e21fc57728f07248adbc946