[ADT] Micro-optimize the Triple constructor by doing a single split and

[ADT] Micro-optimize the Triple constructor by doing a single split and
re-using the resulting components rather than repeatedly splitting and
re-splitting to compute each component as part of the initializer list.

This is more work on PR23676. Sadly, it doesn't help much. It removes
the constructor from my profile, but doesn't make a sufficient dent in
the total time. But it should play together nicely with subsequent
changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247250 91177308-0d34-0410-b5e6-96231b3b80d8