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[GlobalISel][AArch64] Adding -disable-gisel-legality-check CL option

Description

[GlobalISel][AArch64] Adding -disable-gisel-legality-check CL option

Currently it's impossible to test InstructionSelect pass with MIR which
is considered illegal by the Legalizer in Assert builds. In early stages
of porting an existing backend from SelectionDAG ISel to GlobalISel,
however, we would have very basic CallLowering, Legalizer, and
RegBankSelect implementations, but rather functional Instruction Select
with quite a few patterns selectable due to the semi-automatic porting
process borrowing them from SelectionDAG ISel.

As we are trying to define legality as a property of being selectable by
the instruction selector, it would be nice to be able to easily check
what the selector can do in its current state w/o the legality check
provided by the Legalizer getting in the way.

It also seems beneficial to have a regression testing set up that would
not allow the selector to silently regress in its support of the MIR not
supported yet by the previous passes in the GlobalISel pipeline.

This commit adds -disable-gisel-legality-check command line option to
llc that disables those legality checks in RegBankSelect and
InstructionSelect passes.

It also adds quite a few MIR test cases for AArch64's Instruction
Selector. Every one of them would fail on the legality check at the
moment, but will select just fine if the check is disabled. Every test
MachineFunction is intended to exercise a specific selection rule and
that rule only, encoded in the MachineFunction's name by the rule's
number, ID, and index of its GIM_Try opcode in TableGen'erated
MatchTable (-optimize-match-table=false).

Reviewers: ab, dsanders, qcolombet, rovka

Reviewed By: bogner

Subscribers: kristof.beyls, volkan, aditya_nandakumar, aemerson,
rengolin, t.p.northover, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D42886