[InstCombine] Relax constraints of uses for exp(X) * exp(Y) -> exp(X + Y)

InstCombine didn't perform the transformations when fmul's operands were
the same instruction because it required to have one use for each of them
which is false in the case. This patch fixes this + adds tests for them
and introduces a new function isOnlyUserOfAnyOperand to check these cases
in a single place.

This patch is a result of discussion in D102574.

Differential Revision: https://reviews.llvm.org/D102698