ARM64: Improve String.equals() intrinsic for const strings.

And add additional tests to 021-string2.

aosp_angler-userdebug:
  before:
    arm64 boot*.oat: 43324664
    arm64 boot*.oat/string compression: 43411112
  after:
    arm64 boot*.oat: 43300136 (-24528)
    arm64 boot*.oat/string compression: 43345464 (-65648)

The string compression code size difference drops from
86448 to 45328.

Test: m test-art-target on Nexus 6P
Test: m test-art-target on Nexus 6P with string compression enabled.
Bug: 31040547
Change-Id: I99a3777b91b248da2b0ac25abd260f9e5abb2c09