[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