Extract out getConstantRangeFromMetadata; NFC

The loop idiom creating a ConstantRange is repeated twice in the
codebase, time to give it a name and a home.

The loop is also repeated in `rangeMetadataExcludesValue`, but using
`getConstantRangeFromMetadata` there would not be an NFC -- the range
returned by `getConstantRangeFromMetadata` may contain a value that none
of the subranges did.

llvm-svn: 251180