[TII] NFCI: Simplify the interface for isTriviallyReMaterializable

Currently `isTriviallyReMaterializable` calls
`isReallyTriviallyReMaterializable` and
`isReallyTriviallyReMaterializableGeneric`. The two interfaces
are confusing, but there are also some real issues with this.

The documentation of this function (see below) suggests that
`isReallyTriviallyRematerializable` allows the target to override the
default behaviour.

  /// For instructions with opcodes for which the M_REMATERIALIZABLE flag is
  /// set, this hook lets the target specify whether the instruction is actually
  /// trivially rematerializable, taking into consideration its operands.

It however implements something different. The default behaviour
is the analysis done in `isReallyTriviallyReMaterializableGeneric`,
which is testing if it is safe to rematerialize the MachineInstr.

The result of `isReallyTriviallyReMaterializable` is only considered if
`isReallyTriviallyReMaterializableGeneric` returns `false`.  That means
there is no way to override the default behaviour if
`isReallyTriviallyReMaterializableGeneric` returns true (i.e. it is safe to
rematerialize, but we'd rather not).

By making this a single interface, we can override the interface to do either.

Reviewed By: craig.topper, nemanjai

Differential Revision: https://reviews.llvm.org/D156520

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