simplify-rtx: Put simplify routines into a class

One of the recurring warts of RTL is that multiplication by a power
of 2 is represented as a MULT inside a MEM but as an ASHIFT outside
a MEM.  It would obviously be better if we didn't have this kind of
context sensitivity, but it would be difficult to remove.

Currently the simplify-rtx.c routines are hard-coded for the
ASHIFT form.  This means that some callers have to convert the
ASHIFTs “back” into MULTs after calling the simplify-rtx.c
routines; see fwprop.c:canonicalize_address for an example.

I think we can relieve some of the pain by wrapping the simplify-rtx.c
routines in a simple class that tracks whether the expression occurs
in a MEM or not, so that no post-processing is needed.

An obvious concern is whether passing the “this” pointer around
will slow things down or bloat the code.  I can't measure any
increase in compile time after applying the patch.  Sizewise,
simplify-rtx.o text increases by 2.3% in default-checking builds
and 4.1% in release-checking builds.

I realise the MULT/ASHIFT thing isn't the most palatable
reason for doing this, but I think it might be useful for
other things in future, such as using local nonzero_bits
hooks/virtual functions instead of the global hooks.

The obvious alternative would be to add a static variable
and hope that it is always updated correctly.

Later patches make use of this.

gcc/
	* rtl.h (simplify_context): New class.
	(simplify_unary_operation, simplify_binary_operation): Use it.
	(simplify_ternary_operation, simplify_relational_operation): Likewise.
	(simplify_subreg, simplify_gen_unary, simplify_gen_binary): Likewise.
	(simplify_gen_ternary, simplify_gen_relational): Likewise.
	(simplify_gen_subreg, lowpart_subreg): Likewise.
	* simplify-rtx.c (simplify_gen_binary): Turn into a member function
	of simplify_context.
	(simplify_gen_unary, simplify_gen_ternary, simplify_gen_relational)
	(simplify_truncation, simplify_unary_operation): Likewise.
	(simplify_unary_operation_1, simplify_byte_swapping_operation)
	(simplify_associative_operation, simplify_logical_relational_operation)
	(simplify_binary_operation, simplify_binary_operation_series)
	(simplify_distributive_operation, simplify_plus_minus): Likewise.
	(simplify_relational_operation, simplify_relational_operation_1)
	(simplify_cond_clz_ctz, simplify_merge_mask): Likewise.
	(simplify_ternary_operation, simplify_subreg, simplify_gen_subreg)
	(lowpart_subreg): Likewise.
	(simplify_binary_operation_1): Likewise.  Test mem_depth when
	deciding whether the ASHIFT or MULT form is canonical.
	(simplify_merge_mask): Use simplify_context.

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