MIPS: Allow uint32 value on optimized frames if they are consumed by safe operations.

MIPS: Allow uint32 value on optimized frames if they are consumed by safe operations.

Port r12367 (27f16c18)

Original commit message:
Safe operations are those that either do not observe unsignedness or have special support for uint32 values:

- all binary bitwise operations: they perform ToInt32 on inputs;
- >> and << shifts: they perform ToInt32 on left hand side and ToUint32 on right hand side;
- >>> shift: it performs ToUint32 on both inputs;
- stores to integer external arrays (not pixel, float or double ones): these stores are "bitwise";
- HChange: special support added for conversions of uint32 values to double and tagged values;
- HSimulate: special support added for deoptimization with uint32 values in registers and stack slots;
- HPhi: phis that have only safe uses and only uint32 operands are uint32 themselves.

BUG=v8:2097
TEST=test/mjsunit/compiler/uint32.js

[palfia, 2012-08-30 22:31:34]

Rebased on r12414

[Sven Panne, 2012-09-03 06:18:54]

LGTM (rubberstamp), landing...