Add donated scratch regs to dependency list only when they are used

[matthewhall2]

Adds msrUsedDonated flag to mark that a donated scratch register has been used.

If using findOrCreateScratchRegister within ICF and there are no free registers, cg->allocateRegister is called causing a fatal assertion
TR_ASSERT_FATAL(false, "Attempting to assign a register (%s) inside ICF", getRegisterName(targetRegister, self()->cg()));
Thus, we have to donate registers before entering an internal control flow region where we request a scratch register.

For example, there are cases where we need at most 2 scratch registers, and the use of the second register is guarded by an if statement (i.e. in the java superclass test on Z):

void genSuperclassTest(..., ScratchRegisterManager *srm, ...) {
    ...
    TR::Register *castClassDepthReg = NULL;
    if (castClassDepth == -1) { // we do not know the depth at compile time
        castClassDepthReg = srm->findOrCreateScratchRegister();
    }
    ....
    TR::Register *objClassDepthReg = srm->findOrCreateScratchRegister();
}

Since the superclass test is generated inside an ICF region, we need to allocate the 2 registers before entering the region. But when castClassDepth != -1, we never use one of the donated registers. Adding this unused register to the dependency list results in an extra real register being allocated.

This change allows the Scratch Register Manager to treat donated and non-donated registers in the same way: they are only added to the dependency list if they are actually used. This way, we allow the user to set the capacity of the scratch register manager to the maximum amount they need, and donate how ever many they want (up to the maximum), without worrying about whether all donated registers are used in all cases.

[matthewhall2]

most tests passing. I've gone through the unstable tests and similar failures were seen in recent nightly builds. Re-running them in Grinder to make sure.
please review @r30shah

[r30shah]

Hi @matthewhall2 , thanks a lot for the change. It is bit unfortunate that in case of checkCast evaluator, you do enter a case where ICF is entered and SRM you have created has not been used to its capacity, causing you to allocate a register inside ICF. I have been thinking about the problem you are having.

Although, the change you are adding here would make it easier to solve problem you are having where SRM is used in checkCast evaluator on Z where you do not need all register in all cases, I do think that a good written evaluator, should ensure that they are allocating by themselves/through SRM only number of registers they are going to use in the evaluator.