arm: Fix up neon_vector_mem_operand [PR97528]

The documentation for POST_MODIFY says:
   Currently, the compiler can only handle second operands of the
   form (plus (reg) (reg)) and (plus (reg) (const_int)), where
   the first operand of the PLUS has to be the same register as
   the first operand of the *_MODIFY.
The following testcase ICEs, because combine just attempts to simplify
things and ends up with
(post_modify (reg1) (plus (mult (reg2) (const_int 4)) (reg1))
but the target predicates accept it, because they only verify
that POST_MODIFY's second operand is PLUS and the second operand
of the PLUS is a REG.

The following patch fixes this by performing further verification that
the POST_MODIFY is in the form it should be.

2020-11-20  Jakub Jelinek  <jakub@redhat.com>

	PR target/97528
	* config/arm/arm.c (neon_vector_mem_operand): For POST_MODIFY, require
	first POST_MODIFY operand is a REG and is equal to the first operand
	of PLUS.

	* gcc.target/arm/pr97528.c: New test.