[InstCombine] fold fdiv with exp/exp2 divisor (PR49147)

Follow-up to:
D96648 / b40fde06
...for the special-case base calls.

From the earlier commit:
This is unusual in the general (non-reciprocal) case because we need
an extra instruction, but that should be better for general FP
reassociation and codegen. We conservatively check for "arcp" FMF
here as we do with existing fdiv folds, but it is not strictly
necessary to have that.