[BOLT] Fix X86MCPlusBuilder::replaceRegWithImm

Reassigning the operand didn't update the operand type which resulted in an
assertion (`Assertion `isReg() && "This is not a register operand!"' failed.`)
Reset the instruction instead.

Test Plan:
```
ninja check-bolt
...
PASS: BOLT-Unit :: Core/./CoreTests/X86/MCPlusBuilderTester.ReplaceRegWithImm/0 (90 of 136)
```

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D120263