Fix bugs in simplified lowering relating to int32/uint32 signs.

Lowering of NumberToUint32 and NumberToInt32 was not correctly accounting for the sign of the input and the sign of the output, emitting the wrong representation changes.

Along the way, I've found cases where MachineOperatorBuilder would break if fed a machine type for loads or stores that was not cached, requiring MachineOperatorBuilder to take zone to allocate operators for these cases.

R=bmeurer@chromium.org, jarin@chromium.org
BUG=

Review URL: https://codereview.chromium.org/714613002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25247 ce2b1a6d-e550-0410-aec6-3dcde31c8c00