[turbofan] More efficient code generation for unsigned modulus.

If we know statically that x and y are both in Unsigned32 or NaN or -0,
and we have SignedSmall or Signed32 feedback for x % y, then we can take
the feedback on the inputs and lower to Uint32Mod.

Drive-by-fix: Refactor this logic into a separate method.

R=jarin@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2287303002
Cr-Commit-Position: refs/heads/master@{#38975}