tree: Fix up get_narrower [PR94724]

tree: Fix up get_narrower [PR94724]

In the recent get_narrower change, I wanted it to be efficient and avoid
recursion if there are many nested COMPOUND_EXPRs.  That builds the
COMPOUND_EXPR nest with the right arguments, but as build2_loc computes some
flags like TREE_SIDE_EFFECTS, TREE_CONSTANT and TREE_READONLY, when it
is called with something that will not be the argument in the end, those
flags are computed incorrectly.
So, this patch instead uses an auto_vec and builds them in the reverse order
so when they are built, they are built with the correct operands.

2020-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/94724
	* tree.c (get_narrower): Instead of creating COMPOUND_EXPRs
	temporarily with non-final second operand and updating it later,
	push COMPOUND_EXPRs into a vector and process it in reverse,
	creating COMPOUND_EXPRs with the final operands.

	* gcc.c-torture/execute/pr94724.c: New test.