x86_64: Implement missing variants of XCHG. Add tests for them.

Also, clean up the handling of a special case when at least one of the
operands for XCHG is RAX/EAX/AX. Add a helper function that deduplicates
the code for different operand sizes.

This patch also extends `EmitOptionalByteRegNormalizingRex32` function
to handle the case when an instruction has both operands in byte
registers, and so it needs REX prefix if either of them is in a special
register (an example of such instruction is `xchg bpl, al`: if only the
source register is checked, no REX would be emitted). Previously
`EmitOptionalByteRegNormalizingRex32` handled only the case when the
source register is special (an example of such instruction is
`movzxb rax, bpl`).

Bug: 65872996
Test: m test-art-host-gtest
Change-Id: I20c5e9375bbd15d799e5748b127d154ddcc0fc11

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