[X86][SSE] Add extra trunc(shl) test cases

The existing trunc_shl_17_v8i16_v8i32 test case should (but doesn't) fold to zero, I've added 2 new test cases:
 - trunc_shl_16_v8i16_v8i32 which folds to zero (this is actually testing the target faux shuffle combine)
 - trunc_shl_15_v8i16_v8i32 which should perform the full shl + truncate

llvm-svn: 334188