[PPC] add extract sig/exp test data class for vec float and vec double.

[PPC] add extract sig/exp test data class for vec float and vec double.

 Add vector extract exponent/significand functions to altivec.h, as well as
 functions (and related constants) to test the data class of vector float
 and vector double.

 Differential Revision: https://reviews.llvm.org/D26271

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@286830 91177308-0d34-0410-b5e6-96231b3b80d8