target/mips: Alias MSA vector registers on FPU scalar registers

target/mips: Alias MSA vector registers on FPU scalar registers

Commits 863f264d10f ("add msa_reset(), global msa register") and
cb269f273fd ("fix multiple TCG registers covering same data")
removed the FPU scalar registers and replaced them by aliases to
the MSA vector registers.

It is not very clear to have FPU registers displayed with MSA
register names, even if MSA ASE is not present.

Instead of aliasing FPU registers to the MSA ones (even when MSA
is absent), we now alias the MSA ones to the FPU ones (only when
MSA is present).

Reviewed-by: Jiaxun Yang <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Jiaxun Yang <[email protected]>
Message-Id: <20201208003702.4088927[email protected]>