LiveVariables will mark instructions with their implicit subregister
uses. However, it will also mark the subregister as an implicit if its
own definition is a subregister of it, i.e. `$r3 = OP val, implicit-def
$r0_r1_r2_r3, ..., implicit $r2_r3`, even if it is otherwise unused,
which defines $r3 on the same line it is used.

This change ensures such uses are marked without implicit, i.e. `$r3 =
OP val, implicit-def $r0_r1_r2_r3, ..., $r2_r3`.

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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>

The test coverage is lacking for many of these errors.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

We can use *Set::insert_range to collapse:

  for (auto Elem : Range)
    Set.insert(E);

down to:

  Set.insert_range(Range);

Update some other places to avoid implicit conversions this introduces,
but I probably missed some.