Use an empty Register() instead of -1U.

Menezes, Evandro, Sebastian Pop, and Aditya Kumar. "Clustering case
statements for indirect branch predictors." arXiv preprint
arXiv:1910.02351 (2019).

https://arxiv.org/pdf/1910.02351v2

This will help re-use this code with the upcoming GlobalISel implementation of
this optimization.

visitJumpTable is called on FinishBasicBlock. At that time, getCurSDLoc
will always return SDLoc without DebugLoc since CurInst was set to
nullptr after visiting each instruction.
This patch passes SDLoc to buildJumpTable when visiting SwitchInst so
that visitJumpTable can use it later.

This will make it easy for callers to see issues with and fix up calls
to createTargetMachine after a future change to the params of
TargetMachine.

This matches other nearby enums.

For downstream users, this should be a fairly straightforward
replacement,
e.g. s/CodeGenOpt::Aggressive/CodeGenOptLevel::Aggressive
or s/CGFT_/CodeGenFileType::

This renames the primary methods for creating a zero value to `getZero`
instead of `getNullValue` and renames predicates like `isAllOnesValue`
to simply `isAllOnes`.  This achieves two things:

1) This starts standardizing predicates across the LLVM codebase,
   following (in this case) ConstantInt.  The word "Value" doesn't
   convey anything of merit, and is missing in some of the other things.

2) Calling an integer "null" doesn't make any sense.  The original sin
   here is mine and I've regretted it for years.  This moves us to calling
   it "zero" instead, which is correct!

APInt is widely used and I don't think anyone is keen to take massive source
breakage on anything so core, at least not all in one go.  As such, this
doesn't actually delete any entrypoints, it "soft deprecates" them with a
comment.

Included in this patch are changes to a bunch of the codebase, but there are
more.  We should normalize SelectionDAG and other APIs as well, which would
make the API change more mechanical.

Differential Revision: https://reviews.llvm.org/D109483

Only include the headers we actually need, and move the remaining includes down to implicit dependent files.

Use forward declarations and move the include down to dependent files that actually use it.

This also exposes a number of implicit dependencies on KnownBits.h

Replace with forward declaration and move dependency down to source files that actually need it.

Both TargetLowering.h and TargetMachine.h are 2 of the most expensive headers (top 10) in the ClangBuildAnalyzer report when building llc.

Summary:
(Split of off D67120)

TargetLowering/TargetTransformationInfo/SwitchLoweringUtils changes for profile
guided size optimization.

Reviewers: davidxl

Subscribers: eraman, hiraditya, haicheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69580