Add support for SEB/SEH instructions, which are introduced in mips32r2.
SEB/SEH can be used to sign-extend byte/halfword in registers directly without passing through memory.
Ref: The MIPS32 Instruction Set, Revision 5.04: https://s3-eu-west-1.amazonaws.com/downloads-mips/documents/MD00086-2B-MIPS32BIS-AFP-05.04.pdf
Updates #60072
Change-Id: I33175ae9d943ead5983ac004bd2a158039046d65
Reviewed-on: https://go-review.googlesource.com/c/go/+/515475
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>