[FLANG]Add maxval simplification support

Add simplifcation pass for MAXVAL intrinsic function

This refactors some of the code to allow variation on the
initialization value and operation performed within the loop,
reusing the majority of code for both SUM and MAXVAL.

Adding tests for the test-cases that produce different output
than the SUM function.

Reviewed By: vzakhari

Differential Revision: https://reviews.llvm.org/D132234