make __builtin_isfinite more efficient (PR27145)

isinf (is infinite) and isfinite should be implemented with the same function
except we change the comparison operator.

See PR27145 for more details:
https://llvm.org/bugs/show_bug.cgi?id=27145

Ref: forked off of the discussion in D18513.

Differential Revision: http://reviews.llvm.org/D18648

llvm-svn: 265675