The main purpose of this commit is to refactor the old switch-based logic we
had for compiling match expressions and use an if-statement-based approach.

So, naturally, the old logic was removed in this commit. That includes
sort_tuple_patterns, simplify_tuple_match, PatternMerge and patterns_mergeable.

In the new approach, each arm is transformed into an if statement. And for
any if statement, there are two main components: the condition and the then_block.

The entity responsible for handling the condition is CompileMatchArmCondition.
It's a class that traverses down the arm pattern and the match scrutinee expression
simultaneously. Whenever it meets a base-case in the pattern (i.e. expression or identifier
but identifiers are not handled for now), it takes the respective field of the scrutinee
and returns a backend comparison expression for them.

As for the then_block, it's compiled by SimplifyMatchExpr::compile_case_expr.
This function transforms the HIR::Expr match case final expression
into a backend block so it can be used easily inside the if_statement. In case
the expr is of type BlockExpr, then we can easily compile it using CompileBlock::compile.
Otherwise, we'll have to create a new block and add the expression as a statement.

Now we have the means of creating the if statements. All that's left is to glue the
multiple match arms (if_statements) together, and that's where
SimplifyMatchExpr::simplify_remaining_cases comes in. It compiles the condition and
then_block with the help of the functions mentioned earlier. As for the else_block,
it should contain the if_statements for the remaining match arms. So
simplify_remaining_cases calculates the else_block recursively by calling itself.
After that it returns the compiled if_statement with the condition, then_block, and
else_block.

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (patterns_mergeable): Removed.
	(struct PatternMerge): Removed.
	(sort_tuple_patterns): Removed.
	(simplify_tuple_match): Removed.
	(check_match_scrutinee): Renamed to ..
	(check_match_expr_type): this
	(CompileExpr::visit): Refactored MatchExpr compilation to an
	if-statement-based approach.
	(SimplifyMatchExpr::go): New function. The entry point for compiling
	a match expression into an if statement
	(SimplifyMatchExpr::compile_case_expr): New function. Compiles the match
	case expr.
	(SimplifyMatchExpr::simplify_remaining_cases): New function. Contains the
	main logic for compiling match expressions.
	* backend/rust-compile-expr.h (class SimplifyMatchExpr): New class.
	* backend/rust-compile-pattern.cc (CompilePatternCaseLabelExpr::visit):
	Mostly renamed to...
	(CompileMatchArmCondition::visit): this. Added a tuple visit.
	* backend/rust-compile-pattern.h (class CompilePatternCaseLabelExpr):
	Mostly renamed to...
	(class CompileMatchArmCondition): this.

gcc/testsuite/ChangeLog:
	* rust/execute/torture/match_tuple2.rs: New test.

Signed-off-by: Mahmoud Mohamed <mahadelr19@gmail.com>