Node::InputCount() and ::InputAt() have to check for inline/out-of-line
inputs every time they are called. The compiler doesn't seem to be very
good at caching the result of this check, meaning that it (and all its
jumps) would happen for every node access.

Previously we would get around this sometimes, by using Node::inputs(),
which returned a Node::Inputs iterable over node inputs. However,
sometimes node access is more convenient using an index, or we also
want to access the count. This patch adds an index accessor and 'count'
method to Node::Inputs, and replaces several uses of InputCount and
InputAt with this accessor.

Review-Url: https://codereview.chromium.org/2617123002
Cr-Commit-Position: refs/heads/master@{#42179}