[turbofan] Reduce shl with sar/shr and same shift amount to bit-and.

The shl(sar(x, K), K) is a common pattern with typed loads/stores in
asm.js.

TEST=compiler-unittests,mjsunit/asm/int32array-unaligned
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/598083007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24238 ce2b1a6d-e550-0410-aec6-3dcde31c8c00