Avoid unnecessary binary search in rewrite_call

[topecongiro]

rewrite_call uses binary search to decide the appropriate width for callee.
However, this is unnecessary when we are passing string as callee to rewrite_call,
since the width of string does not change after calling rewrite against it.
This PR avoids using binary search when callee is string.

We get roughly 0~20% speed up against a large code base with this PR.

[topecongiro]

	master (sec)	PR (sec)	LoC
rustfmt	0.22	0.20	28,000
librustc	1.50	1.25	79,000
libstd	0.72	0.72	82,000

[nrc]

Nice! Good savings.

I've always wondered if it is worth using the binary search at all - if there is some heuristic we could use instead and save some time.