write/line: avoid division if minimum instruction length is 1

[philipc]

Fixes #468

I haven't done any performance measurements, but a similar change was significant when reading.
cc @bjorn3

[coveralls]

Coverage decreased (-0.02%) to 85.472% when pulling 573918d on philipc:issue-468 into 15bea8e on gimli-rs:master.

[bjorn3]

This reduces the time of LineProgram::generate_row into which op_advance is inlined from 0.55% (0.44% self) to 0.24% (0.13% self) when generating the debuginfo for the image crate. For comparison the total amount of time spent on debuginfo generation for that crate with this patch is 1.46% of the total compilation time.

[fitzgen]

Nice!