Having a finite Depth (or recursion limit) for computeKnownBits is very
limiting, but is currently a load-bearing necessity, as all KnownBits
are recomputed on each call and there is no caching. As a prerequisite
for an effort to remove the recursion limit altogether, either using a
clever caching technique, or writing a easily-invalidable KnownBits
analysis, make the Depth argument in APIs in ValueTracking uniformly the
last argument with a default value. This would aid in removing the
argument when the time comes, as many callers that currently pass 0
explicitly are now updated to omit the argument altogether.

For code maintainability -- this may result in cases where we are
applying the optimization where it is not profitable, but those are
likely to be rare.

It is known that for vector whose element fits in i16 will be split and
scalarized in SelectionDag's type legalizer
(see SIISelLowering::getPreferredVectorAction).

LRO attempts to undo the scalarizing of vectors across basic block
boundary and shoehorn Values in VGPRs. LRO is beneficial for operations
that natively work on illegal vector types to prevent flip-flopping
between unpacked and packed. If we know that operations on vector will
be split and scalarized, then we don't want to shoehorn them back to
packed VGPR.

Operations that we know to work natively on illegal vector types usually
come in the form of intrinsics (MFMA, DOT8), buffer store, shuffle, phi
nodes to name a few.

This patch just cleans up the "if" condition.  Further cleanups are
left to subsequent patches.

Fixes: SWDEV-507695

Use references instead of pointers for most state and initialize it all
in the constructor, and similarly for the LiveRegOptimizer class.

Identified with misc-include-cleaner.