128-bit vector integer sign extensions correctly lower to the pmovsx instructions even for debug builds.
This patch removes the builtins and reimplements the _mm_cvtepi*_epi* intrinsics using builtin_shufflevector (to extract the bottom most subvector) and builtin_convertvector (to actually perform the sign extension).
After this I'll add a patch for the removal/upgrade of the pmovsx intrinsics on the llvm side.
Note: AVX2 256-bit vector integer sign extensions currently don't lower correctly in debug builds if they need the __builtin_shufflevector stage (although I could add the ones that don't need this straightaway).