This patch is a combination of two changes that have to be
committed as a single unit:

(1) Try to fold IFN_WHILE_ULTs with constant arguments to a VECTOR_CST
    (which is always possible for fixed-length vectors but is not
    necessarily so for variable-length vectors)

(2) Make the SVE port recognise constants that map to PTRUE VLn,
    which includes those generated by the new fold.

(2) can't be tested without (1) and (1) would be a significant
pessimisation without (2).

The target-specific parts also start moving towards doing predicate
manipulation in a canonical VNx16BImode form, using rtx_vector_builders.

2019-08-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree.h (build_vector_a_then_b): Declare.
	* tree.c (build_vector_a_then_b): New function.
	* fold-const-call.c (fold_while_ult): Likewise.
	(fold_const_call): Use it to handle IFN_WHILE_ULT.
	* config/aarch64/aarch64-protos.h (AARCH64_FOR_SVPATTERN): New macro.
	(aarch64_svpattern): New enum.
	* config/aarch64/aarch64-sve.md (mov<PRED_ALL:mode>): Pass
	constants through aarch64_expand_mov_immediate.
	(*aarch64_sve_mov<PRED_ALL:mode>): Use aarch64_mov_operand rather
	than general_operand as the predicate for operand 1.
	(while_ult<GPI:mode><PRED_ALL:mode>): Add a '@' marker.
	* config/aarch64/aarch64.c (simd_immediate_info::PTRUE): New
	insn_type.
	(simd_immediate_info::simd_immediate_info): New overload that
	takes a scalar_int_mode and an svpattern.
	(simd_immediate_info::u): Add a "pattern" field.
	(svpattern_token): New function.
	(aarch64_get_sve_pred_bits, aarch64_widest_sve_pred_elt_size)
	(aarch64_partial_ptrue_length, aarch64_svpattern_for_vl)
	(aarch64_sve_move_pred_via_while): New functions.
	(aarch64_expand_mov_immediate): Try using
	aarch64_sve_move_pred_via_while for predicates that contain N ones
	followed by M zeros but that do not correspond to a VLnnn pattern.
	(aarch64_sve_pred_valid_immediate): New function.
	(aarch64_simd_valid_immediate): Use it instead of dealing directly
	with PTRUE and PFALSE.
	(aarch64_output_sve_mov_immediate): Handle new simd_immediate_info
	forms.

gcc/testsuite/
	* gcc.target/aarch64/sve/spill_2.c: Increase iteration counts
	beyond the range of a PTRUE.
	* gcc.target/aarch64/sve/while_6.c: New test.
	* gcc.target/aarch64/sve/while_7.c: Likewise.
	* gcc.target/aarch64/sve/while_8.c: Likewise.
	* gcc.target/aarch64/sve/while_9.c: Likewise.
	* gcc.target/aarch64/sve/while_10.c: Likewise.

From-SVN: r274402