This was not used much and there are better ways of writing it.

SwiftErrorValueTracking creates vregs at swifterror use sites and then
connects it with appropriate definitions after instruction selection.
To propagate swifterror values SwiftErrorValueTracking::propagateVRegs
iterates over basic blocks in RPO, but some vregs previously created
at use sites may be located in blocks that became unreachable after
instruction selection. Because of that there will no definition for
such vregs and that may cause issues down the pipeline.

To ensure that all vregs created by the SwiftErrorValueTracking will
be defined propagateVRegs was updated to insert IMPLICIT_DEF at the
beginning of unreachable blocks containing swifterror uses.

Related issue: https://github.com/llvm/llvm-project/issues/59751

Reviewed By: compnerd

Differential Revision: https://reviews.llvm.org/D141053

Now X86ISelLowering doesn't depend on many IR analyses.

llvm-svn: 375320

Force using Register.

One downside is the generated register enums require explicit
conversion.

llvm-svn: 364194

Avoids using a plain unsigned for registers throughoug codegen.
Doesn't attempt to change every register use, just something a little
more than the set needed to build after changing the return type of
MachineOperand::getReg().

llvm-svn: 364191

llvm-svn: 361607