[AArch64] Fix LSE2/LSE128/RCPC3 precedence

D142712 added tests for when both lse2 and lse128 are available, but
in practice there is no way to enable LSE128 without LSE2 from clang:
LSE128 is a v9 only feature and LSE2 has been mandatory since v8.4,
and +/-lse2 can not be specified on the clang command line.

Therefore it makes more sense that lse2+lse128 should emit lse128
instructions, otherwise they will not be emitted at all.

It also makes sense to remove the lse128-only backend tests if that set
of attributes is never set by the frontend.

Differential Revision: https://reviews.llvm.org/D143506