[GlobalISel][InstructionSelect] Removing redundant num operands and nested def operan...

[GlobalISel][InstructionSelect] Removing redundant num operands and nested def operands checks, perf patch 2

This patch continues a series of patches that decrease time spent by
GlobalISel in its InstructionSelect pass by roughly 60% for -O0 builds
for large inputs as measured on sqlite3-amalgamation
(http://sqlite.org/download.html) targeting AArch64.

This commit specifically removes number of operands checks that are
redundant if the instruction's opcode already guarantees that number
of operands (or more), and also avoids any kind of checks on a def
operand of a nested instruction as everything about it was already
checked at its use.

The expected performance implication is about 3% off InstructionSelect
comparing to the baseline (before the series of patches)

This patch also contains a bit of NFC changes required for further
patches in the series.

Every commit planned shares the same Phabricator Review.

Reviewers: qcolombet, dsanders, bogner, aemerson, javed.absar

Reviewed By: qcolombet

Subscribers: rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D44700

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332945 91177308-0d34-0410-b5e6-96231b3b80d8