[InstCombine] Remove hasNoInfs check for pow(C,y) -> exp2(log2(C)*y)

We already check hasNoNaNs and that x is finite and strictly positive.
That only leaves the following special cases (taken from the Linux man
page for pow):

If x is +1, the result is 1.0 (even if y is a NaN).
If the absolute value of x is less than 1, and y is negative infinity, the result is positive infinity.
If the absolute value of x is greater than 1, and y is negative infinity, the result is +0.
If the absolute value of x is less than 1, and y is positive infinity, the result is +0.
If the absolute value of x is greater than 1, and y is positive infinity, the result is positive infinity.

The first case is handled elsewhere, and this transformation preserves
all the others, so there is no need to limit it to hasNoInfs.

Differential Revision: https://reviews.llvm.org/D79409