Add a new builtin-function __builtin_expect_with_probability and
intrinsic llvm.expect.with.probability.
The interface is __builtin_expect_with_probability(long expr, long
expected, double probability).
It is mainly the same as __builtin_expect besides one more argument
indicating the probability of expression equal to expected value. The
probability should be a constant floating-point expression and be in
range [0.0, 1.0] inclusive.
It is similar to builtin-expect-with-probability function in GCC
built-in functions.
Differential Revision: https://reviews.llvm.org/D79830
Zhi Zhuang