[InstCombine] Expand the simplification of pow() into exp2()

Generalize the simplification of `pow(2.0, y)` to `pow(2.0 ** n, y)` for all
scalar and vector 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/D49273

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@341095 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed