This is alternative to #145965 that allows RegisterSDNode to be handled
without making a special case.

Add operators to Register to supporting adding an offset to get
another Register.

Fixes: SWDEV-502134

This function is most often used in range based loops or algorithms
where the iterator is implicitly dereferenced. The dereference returns
an SDNode * of the user rather than SDUse * so users() is a better name.

I've long beeen annoyed that we can't write a range based loop over
SDUse when we need getOperandNo. I plan to rename use_iterator to
user_iterator and add a use_iterator that returns SDUse& on dereference.
This will make it more like IR.

Original commit: 19c8475871faee5ebb06281034872a85a2552675

Multiple 2-stage sanitizer buildbots were reporting failures, the issue
has been addressed separately in 29246a92aee87e86cbc2bb64ee520d7385644f34.

Crashes on ARM builds
https://lab.llvm.org/buildbot/#/builders/85/builds/2548
```
DAGCombiner.cpp:16157: SDValue (anonymous
namespace)::DAGCombiner::visitFREEZE(SDNode *):
Assertion `DAG.isGuaranteedNotToBeUndefOrPoison(R,
false) && "Can't create node that may be
undef/poison!"' failed.
```

Issue #114648

This reverts commit 19c8475871faee5ebb06281034872a85a2552675.

Extend recently-added poison-generating IR flag to codegen as well.

This time with 100% more building unit tests. Original commit message follows.

[NFC] Switch a number of DenseMaps to SmallDenseMaps for speedup (#109417)

If we use SmallDenseMaps instead of DenseMaps at these locations,
we get a substantial speedup because there's less spurious malloc
traffic. Discovered by instrumenting DenseMap with some accounting
code, then selecting sites where we'll get the most bang for our buck.