Expressions like the `e` in `e;` or `e, e2`, whose result is immediately
discarded, should not get a synthetic `CopyValue`. This removes a lot of
redundancy from the IR.
To prevent these expressions from being confused with the expressions
from which they get their result, the predicate
`getInstructionConvertedResultExpression` now suppresses results for
expressions that don't produce their own result. This should fix the
mapping between expressions and IR data-flow nodes.
This change ensures that all `Expr`s (except parentheses) have a
`TranslatedExpr` with a `getResult` that's one of its own instructions,
not an instruction from one of its operands. This means that when we
translate back and forth between `Expr` and `Instruction`, like in
`DataFlow::exprNode`, we will not conflate `e` with `&e` or `... = e`.
There are a few IR APIs that we've found to be confusingly named. This PR renames them to be more consistent within the IR and with the AST API:
`Instruction.getFunction` -> `Instruction.getEnclosingFunction`: This was especially confusing when you'd call `FunctionAddressInstruction.getFunction` to get the function whose address was taken, and wound up with the enclosing function instead.
`Instruction.getXXXOperand` -> `Instruction.getXXX`. Now that `Operand` is an exposed type, we want a way to get a specific `Operand` of an `Instruction`, but more often we want to get the definition instruction of that operand. Now, the pattern is that `getXXXOperand` returns the `Operand`, and `getXXX` is equivalent to `getXXXOperand().getDefinitionInstruction()`.
`Operand.getInstruction` -> `Operand.getUseInstruction`: More consistent with the existing `Operand.getDefinitionInstruction` predicate.
