[AArch64] Rework interface to add constant/offset routines

[AArch64] Rework interface to add constant/offset routines

The port had aarch64_add_offset and aarch64_add_constant routines
that did similar things.  This patch replaces them with an expanded
version of aarch64_add_offset that takes separate source and
destination registers.  The new routine also takes a poly_int64 offset
instead of a HOST_WIDE_INT offset, but it leaves the HOST_WIDE_INT
case to aarch64_add_offset_1, which is basically a repurposed
aarch64_add_constant_internal.  The SVE patch will put the handling
of VL-based constants in aarch64_add_offset, while still using
aarch64_add_offset_1 for the constant part.

The vcall_offset == 0 path in aarch64_output_mi_thunk will use temp0
as well as temp1 once SVE is added.

A side-effect of the patch is that we now generate:

        mov     x29, sp

instead of:

        add     x29, sp, 0

in the pr70044.c test.

2018-01-11  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_force_temporary): Assert that
	x exists before using it.
	(aarch64_add_constant_internal): Rename to...
	(aarch64_add_offset_1): ...this.  Replace regnum with separate
	src and dest rtxes.  Handle the case in which they're different,
	including when the offset is zero.  Replace scratchreg with an rtx.
	Use 2 additions if there is no spare register into which we can
	move a 16-bit constant.
	(aarch64_add_constant): Delete.
	(aarch64_add_offset): Replace reg with separate src and dest
	rtxes.  Take a poly_int64 offset instead of a HOST_WIDE_INT.
	Use aarch64_add_offset_1.
	(aarch64_add_sp, aarch64_sub_sp): Take the scratch register as
	an rtx rather than an int.  Take the delta as a poly_int64
	rather than a HOST_WIDE_INT.  Use aarch64_add_offset.
	(aarch64_expand_mov_immediate): Update uses of aarch64_add_offset.
	(aarch64_expand_prologue): Update calls to aarch64_sub_sp,
	aarch64_allocate_and_probe_stack_space and aarch64_add_offset.
	(aarch64_expand_epilogue): Update calls to aarch64_add_offset
	and aarch64_add_sp.
	(aarch64_output_mi_thunk): Use aarch64_add_offset rather than
	aarch64_add_constant.

gcc/testsuite/
	* gcc.target/aarch64/pr70044.c: Allow "mov x29, sp" too.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256532 138bc75d-0d04-0410-961f-82ee72b054a4