SIMD: fix issue #104

Issues [#104](https://github.com/openhwgroup/corev-binutils-gdb/issues/104),
[#96](https://github.com/openhwgroup/corev-binutils-gdb/issues/96)

cv.{sll,sra,srl}.sci.{h,b}, cv.extract[u].{h,b}, and cv.insert.{h,b}
use 6-bit immediates but sets the top bits as 0. An error has been
added to prevent the full 6-bits being set.

Files Changed:

  * gas/config/tc-riscv.c: Add operands `b81`, `b82`, `b83`, `b84`.
  * opcodes/riscv-dis.c: Likewise.
  * opcodes/riscv-opc.c: Likewise.

  * gas/testsuite/gas/riscv/cv-simd-extract-b-fail.l: Update test.
  * gas/testsuite/gas/riscv/cv-simd-extract-b-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extract-b-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extract-b-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extract-h-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extract-h-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extract-h-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extract-h-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-b-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-b-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-b-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-b-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-h-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-h-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-h-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-extractu-h-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-b-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-b-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-b-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-b-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-h-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-h-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-h-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-insert-h-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-march-xcvsimd.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-march-xcvsimd.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-b-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-b-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-b-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-b-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-h-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-h-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-h-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sll-sci-h-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-b-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-b-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-b-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-b-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-h-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-h-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-h-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-sra-sci-h-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-b-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-b-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-b-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-b-pass.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-h-fail.l: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-h-fail.s: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-h-pass.d: Likewise.
  * gas/testsuite/gas/riscv/cv-simd-srl-sci-h-pass.s: Likewise.