[FileCheck] Avoid capturing group for {{regex}} (#72136)

For `{{regex}}` we don't really need a capturing group, and only add it
to properly handle cases like `{{foo|bar}}`. This is problematic,
because the use of capturing groups makes our regex implementation
slower (we have to go through the "dissect" stage, which can have
quadratic complexity).

Unfortunately, our regex implementation does not support non-capturing
groups like `(?:regex)`. So instead, avoid adding the group entirely if
the regex doesn't contain any alternations.

This causes a slight difference in escaping behavior, where previously
it was possible to write `{{{{}}` and get the same behavior as
`{{\{\{}}`. This will no longer work. I don't think this is a problem,
especially as we recently taught update_analyze_test_checks.py to emit
`{{\{\{}}`, so this shouldn't get introduced in any new tests.

For CodeGen/X86/vector-interleaved-store-i16-stride-7.ll (our slowest
X86 test) this drops FileCheck time from 6s to 5s (the remainder is
spent in a different regex issue). I expect similar speedups in other
tests using a lot of `{{}}`.