simplification to HClampToUint8

simplification to HClampToUint8

* src/hydrogen-instructions.h (HClampToUint8): Don't mark as having
  flexible representation; instead the output is always an Integer32.
  There is no input representation restriction, so we can still perform
  input-specific truncation.

I tested by looking at the --print-code of

var a = PixelArray(1000000)
function fill(a,x) { for (var i=0; i<a.len; i++) a[i] = x; }

Seems to optimize fine both for double and integer inputs.  But perhaps
there is a better test, for which the original code does better, and
this is a bogus patch.

Review URL: http://codereview.chromium.org/7357003
Patch from Andy Wingo <wingo@igalia.com>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8650 ce2b1a6d-e550-0410-aec6-3dcde31c8c00