[InstCombine] Add testcases for (mul (sext x), cst) --> (sext (mul x, cst'))...

[InstCombine] Add testcases for (mul (sext x), cst) --> (sext (mul x, cst')) and (mul (zext x), cst) --> (zext (mul x, cst')) for vectors constants.

If the multiply won't overflow in the original type we can use a smaller mul and sign extend afterwards. We don't currently support this for vector constants.

llvm-svn: 341884