dillon1000/react

All calls to lowerExpression lower to a unique temporary

This is more followup toward deleting Instruction.lvalue. The previous PR 
ensured that all Instruction.lvalue identifiers are only ever assigned to once. 
Now we ensure that `lowerExpression()` is _only_ called via 
`lowerExpressionToTemporary()`, ie we now always lower every single expression 
to a temporary. 

This will make it easier to make lvalue a part of the InstructionValue instead 
of the instruction itself, in follow-up PRs.