[PowerPC] Add intrinsic to convert between ppc_fp128 and fp128

ppc_fp128 and fp128 are both 128-bit floating point types. However, we
can't do conversion between them now, since trunc/ext are not allowed
for same-size fp types.

This patch adds two new intrinsics: llvm.ppc.convert.f128.to.ppcf128 and
llvm.convert.ppcf128.to.f128, to support such conversion.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D109421