Speed up StringLatin1.regionMatchesCI_UTF16

[Eirik Bjørsnøs]

Hi,

This PR continues the effort to speed up case-insensitive string
comparisons, this time tackling comparison of latin1-coded strings with
utf16-coded strings:

https://github.com/openjdk/jdk/pull/12637

This builds on top of #12632, it makes sense to review that one first.

Thanks,
Eirik.
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