R600: Expand MUL for v4i32/v2i32

Fixes piglit test for OpenCL builtin mul24, and allows mad24 to run.

Patch by: Aaron Watry

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Aaron Watry <awatry@gmail.com>

NOTE: This is a candidate for the 3.3 branch.
llvm-svn: 181578