MachineLICM: Merge logic for implicit and explicit definitions.

Anatoly Trosinenko found that when hasSideEffect was set to 0 in the
definition of LOADgotAUTH, MultiSource/Benchmarks/Ptrdist/ks/ks test
from llvm-test-suite started to crash. The issue was traced down to
MachineLICM pass placing LOADgotAUTH right after an unrelated copy to
x16 like rewriting this code:

````
bb.0:
  renamable $x16 = COPY renamable $x12
  B %bb.1

bb.1:
  ...
  /* use $x16 */
  ...
  renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
  /* use $x20 */
  ...
````

like the following:

````
bb.0:
  renamable $x16 = COPY renamable $x12
  renamable $x20 = LOADgotAUTH target-flags(aarch64-got) @some_variable, implicit-def dead $x16, implicit-def dead $x17, implicit-def dead $nzcv
  B %bb.1

bb.1:
  ...
  /* use $x16 */
  ...
  /* use $x20 */
  ...
```

The issue was caused by inconsistent logic between implicit and explicit
operand definitions, where the implicit side was incorrectly skipping
checking RUDefs for dead operands, leading to RuledOut not being set
for the X16 operand.

Because there isn't really a semantic difference between implicit and
explicit operands at this point, let's remove the isImplicit check and
adjust the logic to do the same thing in both cases:

- For implicit operands, we now check and update RUDefs in the same way
  as explicit operands.
- For explicit operands, we now allow dead operands to be skipped.

Reviewers: arsenm, s-barannikov, atrosinenko

Reviewed By: arsenm, s-barannikov

Pull Request: https://github.com/llvm/llvm-project/pull/147624