[IVDescriptor] Get the exact FP instruction that does not allow reordering

This is a bugfix in IVDescriptor.cpp.

The helper function `RecurrenceDescriptor::getExactFPMathInst()`
is supposed to return the 1st FP instruction that does not allow
reordering. However, when constructing the RecurrenceDescriptor,
we trace the use-def chain staring from a PHI node and for each
instruction in the use-def chain, its descriptor overrides the
previous one. Therefore in the final RecurrenceDescriptor we
constructed, we lose previous FP instructions that does not allow
reordering.

Reviewed By: kmclaughlin

Differential Revision: https://reviews.llvm.org/D118073