forwprop: Fix ICE when building a VEC_PERM_EXPR [PR94683]

forwprop: Fix ICE when building a VEC_PERM_EXPR [PR94683]

The type compatibility handling in simplify_vector_constructor is
based on the number of elements and on element type compatibility,
but that's no longer enough to ensure that two vector types are
compatible.  This patch uses a VIEW_CONVERT_EXPR if the permutation
type and result type are distinct.

2020-04-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/94683
	* tree-ssa-forwprop.c (simplify_vector_constructor): Use a
	VIEW_CONVERT_EXPR to handle mixtures of similarly-structured
	but distinct vector types.

gcc/testsuite/
	PR tree-optimization/94683
	* gcc.target/aarch64/sve/acle/general/pr94683.c: New test.