arch-mips: Fix a bug in the MIPS yield instruction.

The yieldThread function implements MIPS's yield instruction, and had a
if condition in it, (src_reg && !yield_mask != 0), which upset clang. When
originally committed, this check read (src_reg & !yield_mask != 0), but
apparently as part of a cleanup sweep a long time ago, it was assumed
that the & was being used as a logical operator and was turned into &&.

Reading the actual description of what the yield instruction is supposed
to do, if src_reg is positive (it is at this point in the function),
then it's supposed to be treated as a bitvector. The YQMask register,
what gets passed in as yield_mask, can have bits set in it which mask
bits that might be set in src_reg, and if any are still set, the an
interrupt should happen, as implemented by the body of the if.

From this description, it's apparent that what the original code was
*trying* to do was to use yield_mask to mask any set bits in src_reg,
and then if any bits were left go into the body. The original author
used ! as a bitwise negating operator since what they *wanted* to do was
to block any bits in src_reg where yield_mask *is* set, and let through
any where yield_mask *is not* set. The & would do that, but only with a
bitwise negated yield_mask. Hence:

if ((src_reg & ~yield_mask) != 0) {
    ...
}

Change-Id: I30d0a47992750adf78c8aa0c28217da187e0cbda
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40957
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Boris Shingarov <shingarov@gmail.com>

src/arch/mips/mt.hh

@@ -273,7 +273,7 @@ yieldThread(TC *tc, Fault &fault, int src_reg, uint32_t yield_mask)
                     curTick(), tc->threadId());
         }
     } else if (src_reg > 0) {
-        if (src_reg && !yield_mask != 0) {
+        if ((src_reg & ~yield_mask) != 0) {
             VPEControlReg vpeControl = tc->readMiscReg(MISCREG_VPE_CONTROL);
             vpeControl.excpt = 2;
             tc->setMiscReg(MISCREG_VPE_CONTROL, vpeControl);