Don't treat variable-length vectors as VLAs during gimplification

Source-level SVE vectors should be gimplified in the same way
as normal fixed-length vectors rather than as VLAs.

This is tested by later SVE patches.

2019-09-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* gimplify.c (gimplify_decl_expr): Use poly_int_tree_p instead
	of checking specifically for INTEGER_CST.

From-SVN: r275870