`utf_char2cells()` calls `utf_printable()` twice (sometimes indirectly,
through `vim_isprintc()`) for characters >= 128. The function can be
refactored to call to it only once.
`utf_printable()` uses binary search on ranges of unprintable characters
to determine if a given character is printable. Since there are only 9
ranges, and the first range contains only one character, binary search
can be replaced with SSE2 SIMD comparisons that check 8 ranges at a
time, and the first range is checked separately. SSE2 is enabled by
default in GCC, Clang and MSVC for x86-64.
Add 3-byte utf-8 to screenpos_spec benchmarks.
