[llvm-reduce] Skip updating calls where OldF isn't the called fn.

When replacing function calls, skip call instructions where the old
function is not the called function, but e.g. the old function is passed
as an argument.

This fixes a crash due to trying to construct invalid IR for the test
case.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D109759