Add support for v1i128 type.

The v1i128 type is needed for the quadword add/substract instructions introduced
in POWER8. Futhermore, the PowerPC ABI specifies that parameters of type v1i128
are to be passed in a single vector register, while parameters of type i128 are
passed in pairs of GPRs. Thus, it is necessary to be able to differentiate
between v1i128 and i128 in LLVM.

http://reviews.llvm.org/D8564

llvm-svn: 235198