regexp: optimize for provably too long inputs

[sylvinus]

As a complement to #31329, we should find out what the maximum length of matchable inputs is at compile-time (when it exists) and return very early.

Some patterns for which this can be computed:

• ^a{2,5}$ (max length 5)
• ^((aaa)|(aa))$ (max length 3)
• ^.$ (max length utf8.UTFMax=4)

[gopherbot]

Change https://golang.org/cl/188800 mentions this issue: regexp: optimize for provably too long inputs