mem_sse2.h: loadu_uint32 -> loadu_int32

this changes the return to int32_t which matches the type with usage
of this call as input to _mm_cvtsi32_si128(), _mm_set_epi32(), etc.
fixes implicit conversion warning with clang-11 -fsanitize=undefined

Change-Id: I1425f12d4f79155dd5d7af0eb00fbdb9f1940544
3 files changed