sra: Cap number of sub-access propagations with a param (PR 93435)

sra: Cap number of sub-access propagations with a param (PR 93435)

PR 93435 is a perfect SRA bomb.  It initializes an array of 16 chars
element-wise, then uses that to initialize an aggregate that consists
of four such arrays, that one to initialize one four times as big as
the previous one all the way to an aggregate that has 64kb.

This causes the sub-access propagation across assignments to create
thousands of byte-sized artificial accesses which are then eligible to
be replaced - they do facilitate forward propagation but there is
enough of them for DSE to never finish.

This patch avoids that situation by accounting how many of such
replacements can be created per SRA candidate.  The default value of
32 was just the largest power of two that did not slow down
compilation of the testcase, but it should also hopefully be big
enough for any reasonable input that might rely on the optimization.

2020-03-20  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/93435
	* params.opt (sra-max-propagations): New parameter.
	* tree-sra.c (propagation_budget): New variable.
	(budget_for_propagation_access): New function.
	(propagate_subaccesses_from_rhs): Use it.
	(propagate_subaccesses_from_lhs): Likewise.
	(propagate_all_subaccesses): Set up and destroy propagation_budget.

	gcc/testsuite/
	* gcc.dg/tree-ssa/pr93435.c: New test.