[X86][SSE] Replace lossless i32/f32 to f64 conversion intrinsics with generic IR

Both the (V)CVTDQ2PD(Y) (i32 to f64) and (V)CVTPS2PD(Y) (f32 to f64) conversion instructions are lossless and can be safely represented as generic __builtin_convertvector calls instead of x86 intrinsics without affecting final codegen.

This patch removes the clang builtins and their use in the sse2/avx headers - a future patch will deal with removing the llvm intrinsics, but that will require a bit more work.

Differential Revision: http://reviews.llvm.org/D20528

llvm-svn: 270499