[X86] VRNDSCALE* folding from masked and scalar ffloor and fceil patterns

This patch handles back-end folding of generic patterns created by lowering the
X86 rounding intrinsics to native IR in cases where the instruction isn't a
straightforward packed values rounding operation, but a masked operation or a
scalar operation.

Differential Revision: https://reviews.llvm.org/D45203

llvm-svn: 335037