aarch64: Fix SVE ACLE handling of SImode pointers

aarch64: Fix SVE ACLE handling of SImode pointers

This long-overdue patch promotes SImode pointers to DImode addresses,
avoiding various ICEs in the existing tests.

2020-01-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-sve-builtins.h
	(function_expander::convert_to_pmode): Declare.
	* config/aarch64/aarch64-sve-builtins.cc
	(function_expander::convert_to_pmode): New function.
	(function_expander::get_contiguous_base): Use it.
	(function_expander::prepare_gather_address_operands): Likewise.
	* config/aarch64/aarch64-sve-builtins-sve2.cc
	(svwhilerw_svwhilewr_impl::expand): Likewise.