[BOLT] BL/BH are considered aliases in regreassign

The relationship of X86 registers is shown in the diagram. BL and BH do
not have a direct alias relationship. However, if the BH register cannot be
swapped, then the BX/EBX/RBX registers cannot be swapped as well, which
means that BL register also cannot be swapped. Therefore, in the presence
of BX/EBX/RBX registers, BL and BH have an alias relationship.

    ┌────────────────┐
    │  RBX           │
    ├────┬───────────┤
    │    │ EBX       │
    ├────┴──┬────────┤
    │       │   BX   │
    ├───────┼───┬────┤
    │       │BH │BL  │
    └───────┴───┴────┘

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D155098