[AArch64] Improve load/store optimizer to handle LDUR + LDR.

This patch allows the mixing of scaled and unscaled load/stores to form
load/store pairs.

This is a reapplication of r259812, which had an incorrect assert.  The
test_stur_str_no_assert() test is a reduced version of the issue hit in
the AArch64 self-host.

PR24465

llvm-svn: 260523