[PowerPC] Do not round values prior to converting to integer

Adding the FP_ROUND nodes when combining FP_TO_[SU]INT of elements
feeding a BUILD_VECTOR into an FP_TO_[SU]INT of the built vector
loses precision. This patch removes the code that adds these nodes
to true f64 operands. It also adds patterns required to ensure
the code is still vectorized rather than converting individual
elements and inserting into a vector.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38342

Differential Revision: https://reviews.llvm.org/D50121

llvm-svn: 338658