expr: Use constant_lower_bound classifying constructor els [PR116595].

expr: Use constant_lower_bound classifying constructor els [PR116595].

In categorize_ctor_elements_1 we do
  VECTOR_CST_NELTS (value).to_constant ()
but VALUE's type can be a VLA vector (since r15-5780-g17b520a10cdaab).

This patch uses constant_lower_bound instead.

	PR middle-end/116595

gcc/ChangeLog:

	* expr.cc (categorize_ctor_elements_1): Use
	constant_lower_bound.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/rvv/autovec/pr116595.C: New test.