We record the usage of each `Predicate` and sort them by usage.
For the top 8 `Predicate`s, we will emit a `PC_CheckPredicateN` to
save one byte.
Overall this reduces the llc binary size with all in-tree targets by
about 61K.
This is a recommit of 1a57927, which was reverted in bc98c31.
The CI failures occurred when doing expensive checks (with option
`LLVM_ENABLE_EXPENSIVE_CHECKS` being ON).
The key point here is that we need stable sorting result in the
test, but doing expensive checks uncovered the non-determinism of
`llvm::sort`. So `llvm::sort` is changed to `llvm::stable_sort`
in this revised patch.
And we use `llvm::MapVector` to keep insertion order.
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