[InstCombine] Don't use getFirstNonPHI in FoldIntegerTypedPHI

getFirstNonPHI iterates over all the instructions in a block until it
finds a non-PHI.

Then, the loop starts from the beginning of the block and goes through
all the instructions until it reaches the instruction found by
getFirstNonPHI.

Instead of doing that, just stop when a non-PHI is found.

This reduces the compile-time of a test case discussed in
https://reviews.llvm.org/D47023 by 13x.

Not entirely sure how to come up with a test case for this since it's a
compile time issue that would significantly slow down running the tests.

Differential Revision: https://reviews.llvm.org/D70016