aarch64: Fix up RTL sharing bug in aarch64_load_symref_appropriately [PR104910]

aarch64: Fix up RTL sharing bug in aarch64_load_symref_appropriately [PR104910]

We unshare all RTL created during expansion, but when
aarch64_load_symref_appropriately is called after expansion like in the
following testcases, we use imm in both HIGH and LO_SUM operands.
If imm is some RTL that shouldn't be shared like a non-sharable CONST,
we get at least with --enable-checking=rtl a checking ICE, otherwise might
just get silently wrong code.

The following patch fixes that by copying it if it can't be shared.

2022-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR target/104910
	* config/aarch64/aarch64.cc (aarch64_load_symref_appropriately): Copy
	imm rtx.

	* gcc.dg/pr104910.c: New test.