[X86][SSE] Replace (V)CVTTPS2DQ and VCVTTPD2DQ truncating (round to zero)...

[X86][SSE] Replace (V)CVTTPS2DQ and VCVTTPD2DQ truncating (round to zero) f32/f64 to i32 with generic IR (clang)

The 'cvtt' truncation (round to zero) conversions can be safely represented as generic __builtin_convertvector (fptosi) calls instead of x86 intrinsics. We already do this (implicitly) for the scalar equivalents.

Note: I looked at updating _mm_cvttpd_epi32 as well but this still requires a lot more backend work to correctly lower (both for debug and optimized builds).

Differential Revision: http://reviews.llvm.org/D20859

llvm-svn: 271436