This follows on from the previous patch, which corrects xx_loadl_32
for misaligned addresses. Calls to xx_loadl_32 in selfguided_sse4.c
are all followed by a zero-extend, so this patch packages the two into
the inlinable functions xx_load_extend_8_16 and xx_load_extend_8_32.

There were also some hand-rolled loads (which matched the old body of
xx_loadl_32 and weren't strictly correct when the pointer was
misaligned). This patch fixes them up to use xx_load_extend_8_32.

BUG=aomedia:912

Change-Id: I9c76dd4f41baa1343149aa9c432218a17df8b415