[InstCombine] Replacing X86-specific rounding intrinsics with generic floor-ceil

This patch replaces calls to X86-specific intrinsics with floor-ceil semantics
with calls to target-independent @llvm.floor.* and @llvm.ceil.* intrinsics. This
doesn't affect the resulting machine code, as those intrinsics are lowered to
the same instructions, but exposes these specific rounding cases to generic
optimizations.

Differential Revision: https://reviews.llvm.org/D48067

llvm-svn: 335039