Add two new internal functions (IFN_CRC, IFN_CRC_REV), to provide faster
CRC generation.
One performs bit-forward and the other bit-reversed CRC computation.
If CRC optabs are supported, they are used for the CRC computation.
Otherwise, table-based CRC is generated.
The supported data and CRC sizes are 8, 16, 32, and 64 bits.
The polynomial is without the leading 1.
A table with 256 elements is used to store precomputed CRCs.
For the reflection of inputs and the output, a simple algorithm involving
SHIFT, AND, and OR operations is used.

Co-authored-by: Joern Rennecke <joern.rennecke@embecosm.com>

gcc/

   * doc/md.texi (crc@var{m}@var{n}4,
   crc_rev@var{m}@var{n}4): Document.
   * expr.cc (generate_crc_table): New function.
   (calculate_table_based_CRC): Likewise.
   (expand_crc_table_based): Likewise.
   (gen_common_operation_to_reflect): Likewise.
   (reflect_64_bit_value): Likewise.
   (reflect_32_bit_value): Likewise.
   (reflect_16_bit_value): Likewise.
   (reflect_8_bit_value): Likewise.
   (generate_reflecting_code_standard): Likewise.
   (expand_reversed_crc_table_based): Likewise.
   * expr.h (generate_reflecting_code_standard): New function declaration.
   (expand_crc_table_based): Likewise.
   (expand_reversed_crc_table_based): Likewise.
   * internal-fn.cc: (crc_direct): Define.
   (direct_crc_optab_supported_p): Likewise.
   (expand_crc_optab_fn): New function
   * internal-fn.def (CRC, CRC_REV): New internal functions.
   * optabs.def (crc_optab, crc_rev_optab): New optabs.

Signed-off-by: Mariam Arutunian <mariamarutunian@gmail.com>