* Add initial support for code generation with SSE4.1 instructions. The
following operations are affected:
 - multiplication with v4i32
 - select
 - insertelement
 - extractelement

* Add appropriate lit checks for SSE4.1 instructions. Run the crosstests
in both SSE2 and SSE4.1 mode.

* Introduce the -mattr flag to llvm2ice to control which instruction set
gets used.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/427843002