The aim of this PR is twofold:

- Reduce the number of highly similar TMP|VAR handlers
- Avoid ZVAL_DEREF in most of these cases

This is achieved by guaranteeing that all zend_compile_expr() calls, as well as
all other compile calls with BP_VAR_{R,IS}, will result in a TMP variable. This
implies that the result will not contain an IS_INDIRECT or IS_REFERENCE value,
which was mostly already the case, with two exceptions:

- Calls to return-by-reference functions. Because return-by-reference functions
  are quite rare, this is solved by delegating the DEREF to the RETURN_BY_REF
  handler, which will examine the stack to check whether the caller expects a
  VAR or TMP to understand whether the DEREF is needed. Internal functions will
  also need to adjust by calling the zend_return_unwrap_ref() function.

- By-reference assignments, including both $a = &$b, as well as $a = [&$b]. When
  the result of these expressions is used in a BP_VAR_R context, the reference
  is unwrapped via a ZEND_QM_ASSIGN opcode beforehand. This is exceptionally
  rare.

Closes GH-20628