[modula2] Variable analysis understands DISPOSE and NIL

This patch allows the uninitialized variable analysis to detect pointer
through NIL and incorrectly reusing a pointer after a call to DISPOSE.

gcc/m2/ChangeLog:

	* gm2-compiler/M2Quads.mod (BuildRealFuncProcCall): Set the trash
	parameter value to NIL if DEALLOCATE is detected.
	* gm2-compiler/M2SymInit.mod (CheckDeferredRecordAccess): Pass
	tok to SetVarInitialized.  Pass tok to GetVarComponentInitialized.
	(ComponentFindVar): Add tok parameter.  Check aliased pointer
	against Nil and generate warning if necessary.
	(deRefComponent): Add tok and sym parameters and pass them to
	getContent.
	(SetVarComponentInitialized): Add tok parameter.  Pass tok to
	ComponentFindVar.  Pass tok and sym to deRefComponent.
	(GetVarComponentInitialized): Add tok parameter.  Pass tok to
	ComponentFindVar.  Pass tok to deRefComponent.
	(SetVarInitialized): Add tok parameter.  Pass tok to
	SetVarComponentInitialized.
	(doGetVarInitialized): Add tok parameter.  Pass tok to
	GetVarComponentInitialized.
	(CheckXIndr): Pass lhs and lhstok to getContent.
	(CheckIndrX): Pass rhs and rhstok to getContent.
	(CheckBecomes): Pass destok to ComponentFindVar.  Pass des and
	destok to deRefComponent.
	(CheckAddr): Pass contenttok to GetVarInitialized.  Pass ptrtok
	to SetVarInitialized.
	(CheckReadBeforeInitQuad): Pass op1tok to SetVarInitialized for
	op1 cases and op3tok for op3 cases.
	(trashParam): Get operand tokens.  Pass op3tok to
	SetVarInitialized.  Pass op3 and op3tok to getContent.
	Alias ptr to NIL if procedure is DEALLOCATE.  Pass op3tok to
	SetVarInitialized.
	(IsDeallocate): New procedure function.
	(DetectTrash): Use IsDeallocate.
	(SetupLAlias): Allow exp to be Nil.
	(getContent): Generate warning message if ptr is Nil.

gcc/testsuite/ChangeLog:

	* gm2/switches/uninit-variable-checking/procedures/fail/testdispose.mod: New test.
	* gm2/switches/uninit-variable-checking/procedures/fail/testdispose2.mod: New test.
	* gm2/switches/uninit-variable-checking/procedures/fail/testnil.mod: New test.

Signed-off-by: Gaius Mulley <gaiusmod2@gmail.com>

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