[llvm-mca] Add support for instructions with a variadic number of operands.

By default, llvm-mca conservatively assumes that a register operand from the
variadic sequence is both a register read and a register write.  That is because
MCInstrDesc doesn't describe extra variadic operands; we don't have enough
dataflow information to tell which register operands from the variadic sequence
is a definition, and which is a use instead.

However, if a variadic instruction is flagged 'mayStore' (but not 'mayLoad'),
and it has no 'unmodeledSideEffects', then llvm-mca (very) optimistically
assumes that any register operand in the variadic sequence is a register read
only. Conversely, if a variadic instruction is marked as 'mayLoad' (but not
'mayStore'), and it has no 'unmodeledSideEffects', then llvm-mca optimistically
assumes that any extra register operand is a register definition only.
These assumptions work quite well for variadic load/store multiple instructions
defined by the ARM backend.

llvm-svn: 347522