[TableGen] Emit separate computeRequiredFeatures() function

A function is already emitted in *GenInstrInfo.inc that takes Opcode
number and a set of supported Features and reports fatal error if some
of the required features are missing.

The information about features required by the particular opcode can be
reused by llvm-exegesis, so move its computation info a separate
computeRequiredFeatures() function. Then verifyInstructionPredicates()
can just compare the sets of available and required features computed by
the other functions.

This commit moves the definition of FeatureBitsets[] as well as CEFBS_*
enumerator values (that are indices into FeatureBitsets[] array) inside
the computeRequiredFeatures() function because these are implementation
details of that function. The inclusion of potentially huge
computeRequiredFeatures() function is now controlled by a dedicated
macro that is set for simplicity by TableGen-erated code itself if
`defined(ENABLE_INSTR_PREDICATE_VERIFIER) && !defined(NDEBUG)`.

~~

Huawei RRI, OS Lab

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D148516

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