[LV][IVDescriptors] Fix recurrence identity element for FMin and FMax reductions

For a min and max reduction idioms, the identity (i.e. neutral) element
should be datatype's highest and lowest possible values respectively.
Current implementation in IVDescriptors incorrectly returns -Inf for FMin
reduction and +Inf for FMax reduction. This patch fixes this bug which
was causing incorrect reduction computation results in loops vectorized
by LV.

Differential Revision: https://reviews.llvm.org/D137220