[llvm] [MachineInstr] add insert method for variadic instructions (PR #67699)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

Spooky, looks like my newly added tests are failing in presubmit.

When I run them locally via:
```
$ ./llvm/build/unittests/CodeGen/CodeGenTests --gtest_filter=MachineInstrTest.SpliceOperands
```
I observe no issues.  When I run them via:
```
$ ./llvm/build/unittests/CodeGen/CodeGenTests
```
sometimes I observe a segfault, sometimes I observe
```
CodeGenTests: /android0/llvm-project/llvm/include/llvm/CodeGen/MachineOperand.h:370: Register llvm::MachineOperand::getReg() const: Assertion `isReg() && "This is not a register operand!"' failed.
```
Is there some kind of race on MachineRegisterInfo for these tests?

https://github.com/llvm/llvm-project/pull/67699