The number of actual arguments should always be available, there's no
point in trying to optimize away a simple assignment of an immediate to
a register before some calls.

The main motivation is to have a consistent state at the beginning of every
function. Currently the arguments register (i.e. rax or eax) either contains
the number of arguments or some random garbage depending on whether
the callsite decided that the callee might need the information or not.
This causes trouble with runtime implementations of functions that
do not set internal_formal_parameter_count to the DontAdaptArguments
sentinel (we don't have any of those yet), but also makes it impossible
to sanity check the arguments in the callee, because the callee doesn't
know whether the caller decided to pass the number of arguments or
random garbage.

BUG=v8:4413
LOG=n

Review URL: https://codereview.chromium.org/1330033002

Cr-Commit-Position: refs/heads/master@{#30648}