[ARC] Add more load/store variants.

On ARC ISA, general format of load instruction is this:

    LD<zz><.x><.aa><.di> a, [b,c]
And general format of store is this:
    ST<zz><.aa><.di> c, [b,s9]
Where:

<zz> is data size field and can be one of
  <empty> (bits 00) - Word (32-bit), default behavior
  B             (bits 01) - Byte
  H             (bits 10) - Half-word (16-bit)

 <.x> is data extend mode:
  <empty> (bit 0) - If size is not Word(32-bit), then data is zero extended
  X       (bit 1) - If size is not Word(32-bit), then data is sign extended

 <.aa> is address write-back mode:
  <empty> (bits 00) - no write-back
  .AW  (bits 01) - Preincrement, base register updated pre memory transaction
  .AB  (bits 10) - Postincrement, base register updated post memory transaction

 <.di> is cache bypass mode:
  <empty> (bit 0) - Cached memory access, default mode
  .DI     (bit 1) - Non-cached data memory access

  This patch adds these load/store instruction variants to the ARC backend.

Patch By Denis Antrushin! <denis@synopsys.com>

Differential Revision: https://reviews.llvm.org/D58980

llvm-svn: 356200