Don't build the same matrix twice
Don't build the same matrix twice

The exact same logic was used in check_arms and check_match to build the
matrix of relevant patterns. It would actually probably have been a bug
if it was not the case, since exhaustiveness checking should be the same
as checking reachability of an additional `_ => ...` match branch.

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust