Avoid creating (const (reg ...)) [PR108603]

convert_memory_address_addr_space_1 has two modes: one in which it
tries to create a self-contained RTL expression (which might fail)
and one in which it can emit new instructions where necessary.

When handling a CONST, the function recurses into the CONST's
operand and then constifies the result.  But that's only valid if
the result is still a self-contained expression.  If new instructions
have been emitted, the expression will refer to the (non-constant)
results of those instructions.

In the PR, this caused us to emit a nonsensical (const (reg ...))
REG_EQUAL note.

gcc/
	PR tree-optimization/108603
	* explow.cc (convert_memory_address_addr_space_1): Only wrap
	the result of a recursive call in a CONST if no instructions
	were emitted.

gcc/testsuite/
	PR tree-optimization/108603
	* gcc.target/aarch64/sve/pr108603.c: New test.