instructionWaitsForSGPRWrites function covers ALL SALU instructions,
including those like s_waitcnt that don't read from sgpr. This results
in removing delay_alu instructions in cases like VALU->SGPR->VALU, which
results in performance regression. Change modifies the function so that
it checks if instruction also reads a sgpr.

When a node is picked, we should reset its next cluster candidate to
null before releasing its successors/predecessors.

Unused sdst writing to null to avoid a false VALU->SALU dependency
stall. This requires using the VOP3 encoding.

We have a VALU->SGPR->SALU (VALU writing to SGPR and SALU reading from
it). When VALU is issued, it increments internal counter VA_SDST used to
track use of this SGPR. SALU will not issue until VA_SDST is zero, that
is when VALU is finished writing. Therefore, delays added by s_delay_alu
are not needed in this situation.

This reverts commit 71582c6667a6334c688734cae628e906b3c1ac1d.

Multiple buildbot failures have been reported:
https://github.com/llvm/llvm-project/pull/127212

We have a VALU->SGPR->SALU (VALU writing to SGPR and SALU reading from
it). When VALU is issued, it increments internal counter VA_SDST used to
track use of this SGPR. SALU will not issue until VA_SDST is zero, that
is when VALU is finished writing. Therefore, delays added by s_delay_alu
are not needed in this situation.

Providing the correct operand index allows addPhysRegDataDeps to compute
the correct latency.

Pull Request: https://github.com/llvm/llvm-project/pull/123541

Given the rest of the pass just gives up when it needs to compose
subregisters, folding a subregister extract directly into a reg_sequence
is counterproductive. Later fold attempts in the function will give up
on the subregister operand, preventing looking up through the reg_sequence.

It may still be profitable to do these folds if we start handling
the composes. There are some test regressions, but this mostly
looks better.