The bug was first spotted in the optimized gl_Color4ub() from spec2k's mesa.  The lowering sequences for fptosi and fptoui with i8 or i16 include "mov T_2, T_1" where T_1 and T_2 may have different integer types, and the statement:
      T_2->setPreferredRegister(T_1, true);

If T_2's type is i8 and T_1 is assigned a register that has no 8-bit version, then T_2 gets an unsuitable register.

The fix is to honor RegisterOverlap only when RegMask allows.

It's hard to construct a good test for this, since it depends heavily on register allocation decisions, which will change over time.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/544713002