forwprop: Fix ICE when building an identity constructor [PR94700]

forwprop: Fix ICE when building an identity constructor [PR94700]

This is really PR94683 part 2, handling the case in which the vector is
an identity and so doesn't need a VEC_PERM_EXPR.  I should have realised
at the time that the other arm of the "if" would need the same fix.

2020-04-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/94700
	* tree-ssa-forwprop.c (simplify_vector_constructor): When processing
	an identity constructor, use a VIEW_CONVERT_EXPR to handle mixtures
	of similarly-structured but distinct vector types.

gcc/testsuite/
	PR tree-optimization/94700
	* gcc.target/aarch64/sve/acle/general/pr94700.c: New test.