We were incorrectly changing -1 to 0 for unsigned compares in case 1.
The comment incorrectly said UINT64_MAX is bigger than INT64_MAX, but
we were doing a signed compare and UINT64_MAX is smaller than INT64_MAX
in signed space.
Prevent changing 0 constants since we can use x0. The test cases
for these are contrived to use addi rd, x0, 0. We're more likely
to have a COPY from x0 which we already don't optimize for other
reasons.
Check if registers are virtual before calling hasOneUse. The use count
for physical registers is meaningless.
48a21e6915