[RegisterBankInfo] Avoid code duplication in OperandsMapper for the...

[RegisterBankInfo] Avoid code duplication in OperandsMapper for the computation of the end of range.

Refactor the code so that we do not compute in two different places the
end iterator for the range of new virtual registers for a given operand.
Although this refactoring was intended as NFC, this is not the case
because it actually fixes a bug where we were returning a range off by 1
(too long). Right now, this could not result in an actual bug because we
were accessing this range via the BreakDown size of the related operand.

llvm-svn: 272208