Fix cv.shuffle.b.sci immediate operand

Issue [#68](https://github.com/openhwgroup/corev-gcc/issues/68)

The built-in for `cv.shuffle.b.sci` takes an 8-bit immediate operand.
The top two bits decides which `cv.shufflei*` instruction is used.
The other six bits are printed.

Files Changed:

  * config/riscv/constraints.md: Added new constraints to check
    the top two bits.
  * config/riscv/corev.md: Likewise.
  * config/riscv/riscv.cc: Added new operand type for printing the
    bottom six bits.
  * testsuite/gcc.target/riscv/cv-simd-shufflei0-sci-b-compile-1.c: Updated test.
  * testsuite/gcc.target/riscv/cv-simd-shufflei1-sci-b-compile-1.c: Likewise.
  * testsuite/gcc.target/riscv/cv-simd-shufflei2-sci-b-compile-1.c: Likewise.
  * testsuite/gcc.target/riscv/cv-simd-shufflei3-sci-b-compile-1.c: Likewise.
  * testsuite/gcc.target/riscv/cv-xcvsimd-march-compile-1.c: Likewise.