[InstCombine] pow(C,x) -> exp2(log2(C)*x)

[InstCombine] pow(C,x) -> exp2(log2(C)*x)

Summary:
Transform
pow(C,x)

To
exp2(log2(C)*x)

if C > 0, C != inf, C != NaN (and C is not power of 2, since we have some fold for such case already).

log(C) is folded by the compiler and exp2 is much faster to compute than pow.

Reviewers: spatel, efriedma, evandro

Reviewed By: evandro

Subscribers: lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64099

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365637 91177308-0d34-0410-b5e6-96231b3b80d8