This patch adds a class that uses SSA construction, with debug values as
definitions, to determine whether and which debug values for a particular
variable are live at each point in an IR function. This will be used by the
IR reader of llvm-debuginfo-analyzer to compute variable ranges and
coverage, although it may be applicable to other debug info IR analyses.

Hoist out from LVDWARFReader and LVBinaryReader some generic
code, so it can be available to other readers that do not share the
binary format.

This PULL REQUEST implements vXi8 ctlz lowering for X86 using GFNI instructions

Fixes #140729

The class "ClauseVal" actually represents a definition of an enumeration
value, and in itself it is not bound to any clause. Rename it to EnumVal
and add a comment clarifying how it's translated into an actual enum
definition in the generated source code.

There is no change in functionality.

Introduced by accident in #138299
(https://lab.llvm.org/buildbot/#/builders/164/builds/10604)

On AVX2+ targets these are often used to splat subvectors.

This reverts commit 58cc1675ec7b4aa5bc2dab56180cb7af1b23ade5.

I also made the incorrect assumption that we know both values are
+/-0.0 here as well. Revert for now.

This reverts commit 54da543a14da6dd0e594875241494949cb659b08.

I made a logic error here with the assumption that both values
are known to be +/-0.0.

v2i128 bitreverse previously wasn't being scalarized as it should be.
Also added tests for i128 and v2i128 bitreverse

Before this patch, InstCombine hung because it replaced a value with a
more complex one:
```
%sel = select i1 %cmp, i32 %smax, i32 0 ->
%sel = select i1 %cmp, i32 %masked, i32 0 ->
%sel = select i1 %cmp, i32 %smax, i32 0 ->
...
```
This patch makes this replacement more conservative. It only performs
the replacement iff the new value is one of the operands of the original
value.

Closes https://github.com/llvm/llvm-project/issues/142405.