[IR] Fix replaceUsesWithIf ponetial issue with constants

There can be a use after free in the Value::replaceUsesWithIf()
if two uses point to the same constant. Patch defers handling
of the constants past the iterator scan.

Another potential issue is that handleOperandChange updates all
the uses in a given Constant, not just the one passed to
ShouldReplace. Added a FIXME comment.

Both issues are not currently exploitable as the only use of
this call with constants avoids it.

Differential Revision: https://reviews.llvm.org/D105061