target/mips: Fix TLBWI shadow flush for EHINV,XI,RI

Writing specific TLB entries with TLBWI flushes shadow TLB entries
unless an existing entry is having its access permissions upgraded. This
is necessary as software would from then on expect the previous mapping
in that entry to no longer be in effect (even if QEMU has quietly
evicted it to the shadow TLB on a TLBWR).

However it won't do this if only EHINV, XI, or RI bits have been set,
even if that results in a reduction of permissions, so add the necessary
checks to invoke the flush when these bits are set.

Fixes: 2fb58b73746e ("target-mips: add RI and XI fields to TLB entry")
Fixes: 9456c2fbcd82 ("target-mips: add TLBINV support")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Yongbok Kim <yongbok.kim@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Tested-by: Yongbok Kim <yongbok.kim@imgtec.com>
[yongbok.kim@imgtec.com:
  cosmetic changes]
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
