When iterating over a block, meta instructions have no effect on wait counts,
but their presence drops the reference to earlier waitcnt instructions before
they are processed. This results in spurious wait counts, which do not affect
correctness, but are also not required in the resulting program. Skipping meta
instructions as soon as they are seen cleans this up.

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.

Part of #66838.

https://lab.llvm.org/buildbot/#/builders/52/builds/404
https://lab.llvm.org/buildbot/#/builders/55/builds/358
https://lab.llvm.org/buildbot/#/builders/164/builds/518

This reverts commit ded956440739ae326a99cbaef18ce4362e972679.

Change-Id: Ia0d11b79b8302e79247fe193ccabc0dad2d359a0

The issue is uncovered by #47698: for IR files without a target triple,
-mtriple= specifies the full target triple while -march= merely sets the
architecture part of the default target triple, leaving a target triple which
may not make sense, e.g. riscv64-apple-darwin.

Therefore, -march= is error-prone and not recommended for tests without a target
triple. The issue has been benign as we recognize $unknown-apple-darwin as ELF instead
of rejecting it outrightly.

Differential Revision: https://reviews.llvm.org/D148041