aarch64: Fix aarch64_expand_subvti constant handling [PR93335]

aarch64: Fix aarch64_expand_subvti constant handling [PR93335]

The two patterns that call aarch64_expand_subvti ensure that {low,high}_in1
is a register, while {low,high}_in2 can be a register or immediate.
subdi3_compare1_imm uses the aarch64_plus_immediate predicate for its last
two operands (the value and negated value), but aarch64_expand_subvti calls
it whenever low_in2 is a CONST_INT, which leads to ICEs during vregs pass,
as the emitted insn is not recognized as valid subdi3_compare1_imm.
The following patch fixes that by only using subdi3_compare1_imm if it is ok
to do so, and otherwise force the constant into register and use the
non-immediate version - subdi3_compare1.
Furthermore, previously the code was calling force_reg on high_in2 only if
low_in2 is CONST_INT, on the (reasonable) assumption is that only if low_in2
is a CONST_INT, high_in2 can be non-REG, but with the above changes even in
the else we might have CONST_INT and force_reg doesn't do anything if the
operand is already a REG, so this patch calls it unconditionally.

2020-01-22  Jakub Jelinek  <jakub@redhat.com>

	PR target/93335
	* config/aarch64/aarch64.c (aarch64_expand_subvti): Only use
	gen_subdi3_compare1_imm if low_in2 satisfies aarch64_plus_immediate
	predicate, not whenever it is CONST_INT.  Otherwise, force_reg it.
	Call force_reg on high_in2 unconditionally.

	* gcc.c-torture/compile/pr93335.c: New test.