[Inline] Improving `getExpectedNumberOfCompare` for scenarios with no default branch.

[dianqk]

The compare instruction count should be less than the branch count when default branch is undefined: https://llvm.godbolt.org/z/x6ETdfY79.

Related code:

llvm-project/llvm/lib/Analysis/InlineCost.cpp

Lines 524 to 541 in fd3e7e3

	// Considering forming a binary search, we should find the number of nodes
	// which is same as the number of comparisons when lowered. For a given
	// number of clusters, n, we can define a recursive function, f(n), to find
	// the number of nodes in the tree. The recursion is :
	// f(n) = 1 + f(n/2) + f (n - n/2), when n > 3,
	// and f(n) = n, when n <= 3.
	// This will lead a binary tree where the leaf should be either f(2) or f(3)
	// when n > 3. So, the number of comparisons from leaves should be n, while
	// the number of non-leaf should be :
	// 2^(log2(n) - 1) - 1
	// = 2^log2(n) * 2^-1 - 1
	// = n / 2 - 1.
	// Considering comparisons from leaf and non-leaf nodes, we can estimate the
	// number of comparisons in a simple closed form :
	// n + n / 2 - 1 = n * 3 / 2 - 1
	int64_t getExpectedNumberOfCompare(int NumCaseCluster) {
	return 3 * static_cast<int64_t>(NumCaseCluster) / 2 - 1;
	}

Perhaps we can do these things:

1. Reduce one comparison for the default branch that is undefined.
2. The number of comparison instructions can be linear or even fewer.