[Statepoint] Add getters to StatepointOpers.

To simplify future work on statepoint representation, hide
direct access to statepoint field indices and provide getters
for them. Add getters for couple more statepoint fields.

This also fixes two bugs in MachineVerifier for statepoint:
First, the `break` statement was falling out of `if` statement
scope, thus disabling following checks.
Second, it was incorrectly accessing some fields like CallingConv -
StatepointOpers gives index to their value directly, not to
preceeding field type encoding.

Reviewed By: skatkov
Differential Revision: https://reviews.llvm.org/D78119