Prefer to use 'rd' as a scratch.

This is generally useful, but in particular reduces scratch register
pressure in code sequences using ComputeAddress. For example:

    MemOperand addr(...);
    UseScratchRegisterScope temps(&masm);
    Register computed = temps.AcquireX();
    __ ComputeAddress(computed, addr);

Before this patch, that sequence usually required two scratch registers;
one for `computed`, and one for immediate synthesis inside
`ComputeAddress`. With this patch, the same code sequence only needs one
scratch register.

Change-Id: I9c93e6cab51bdacf36046d4d770dc81d1a65a34c