[X86] Fix MCSymbolizer interface for X86Disassembler

Fix a number of issues with MCSymbolizer::tryAddingSymbolicOperand()
in X86Disassembler:

  * Pass instruction size instead of immediate size.
  * Correctly adjust the value of PC-relative operands.
  * Set operand offset to zero when the operand is specified
    implicitly.

Reviewed By: Amir, skan

Differential Revision: https://reviews.llvm.org/D121065