Handle VIEW_CONVERT_EXPR for variable-length vectors

Handle VIEW_CONVERT_EXPR for variable-length vectors

This patch handles VIEW_CONVERT_EXPRs of variable-length VECTOR_CSTs
by adding tree-level versions of native_decode_vector_rtx and
simplify_const_vector_subreg.  It uses the same code for fixed-length
vectors, both to get more coverage and because operating directly on
the compressed encoding should be more efficient for longer vectors
with a regular pattern.

The structure and comments are very similar between the tree and
rtx routines.

2019-11-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* fold-const.c (native_encode_vector): Turn into a wrapper function,
	splitting the main code out into...
	(native_encode_vector_part): ...this new function.
	(native_decode_vector_tree): New function.
	(fold_view_convert_vector_encoding): Likewise.
	(fold_view_convert_expr): Use it for converting VECTOR_CSTs
	to VECTOR_TYPEs.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/temporaries_1.c: New test.

From-SVN: r278410