[InlineCost] model calls to llvm.is.constant* more carefully

llvm.is.constant* intrinsics are evaluated to 0 or 1 integral values.

A common use case for llvm.is.constant comes from the higher level
__builtin_constant_p. A common usage pattern of __builtin_constant_p in
the Linux kernel is:

    void foo (int bar) {
      if (__builtin_constant_p(bar)) {
        // lots of code that will fold away to a constant.
      } else {
        // a little bit of code, usually a libcall.
      }
    }

A minor issue in InlineCost calculations is when `bar` is _not_ Constant
and still will not be after inlining, we don't discount the true branch
and the inline cost of `foo` ends up being the cost of both branches
together, rather than just the false branch.

This leads to code like the above where inlining will not help prove bar
Constant, but it still would be beneficial to inline foo, because the
"true" branch is irrelevant from a cost perspective.

For example, IPSCCP can sink a passed constant argument to foo:

...