native_powr: Switch implementation to native_exp2 and native_log2

v2: don't use assume
    check only for x<0, the other conditions are handled transparently
v3: don't check inputs at all, nan propagation works as expected

Reviewer: Jeroen Ketema
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 318204