[InstCombine] For pow(x, +/-0.5), stop falling into pow(x, 1.5), etc. case

The current code for handling pow(x, y) where y is an integer plus 0.5
is not explicitly guarded against attempting to transform the case where
abs(y) is exactly 0.5.

The latter case is meant to be handled by `replacePowWithSqrt`. Indeed,
if the pow(x, integer+0.5) case proceeds past a certain point, it will
hit an assertion by attempting to form pow(x, 0) using `getPow`.

This patch adds an explicit check to prevent attempting the
pow(x, integer+0.5) transformation on pow(x, +/-0.5) as suggested during
the review of D87877. This has the effect of retaining the shrinking of
`pow` to `powf` when the `sqrt` libcall cannot be formed.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D88066