Add method for replacing instructions to LibCallSimplifier

In some cases the library call simplifier may need to replace instructions
other than the library call being simplified.  In those cases it may be
necessary for clients of the simplifier to override how the replacements
are actually done.  As such, a new overrideable method for replacing
instructions was added to LibCallSimplifier.

A new subclass of LibCallSimplifier is also defined which overrides
the instruction replacement method.  This is because the instruction
combiner defines its own replacement method which updates the worklist
when instructions are replaced.

llvm-svn: 167681