[InstCombine] Expand the simplification of pow() with nested exp{,2}()

Expand the simplification of `pow(exp{,2}(x), y)` to all FP types.

This improvement helps some benchmarks in SPEC CPU2000 and CPU2006, such as
252.eon, 447.dealII, 453.povray.  Otherwise, no significant regressions on
x86-64 or A64.

Differential revision: https://reviews.llvm.org/D51195

llvm-svn: 340948