[X86][MC] Fix the bug of -output-asm-variant=1 for intel syntax

Before this patch

```
$ echo "leal    (,%r15), %eax" | llvm-mc --show-encoding --output-asm-variant=1

        lea     eax, [r15]                      # encoding: [0x42,0x8d,0x04,0x3d,0x00,0x00,0x00,0x00]

$ echo "lea     eax, [r15]" | llvm-mc --show-encoding -x86-asm-syntax=intel --output-asm-variant=1

        lea     eax, [r15]                      # encoding: [0x41,0x8d,0x07]
```

MC printed the register r15 as a base in intel syntax even when it's an index.
Then we got a different encoding by using the assembly from the output of the
first command.

I believe the behavior is too weird to be called a feature.

After this patch, we get

```
$ echo "leal    (,%r15), %eax" | llvm-mc --show-encoding --output-asm-variant=1

        lea     eax, [1*r15]                    # encoding: [0x42,0x8d,0x04,0x3d,0x00,0x00,0x00,0x00]
```

Reviewed By: RKSimon, pengfei, MaskRay

Differential Revision: https://reviews.llvm.org/D144183