Implement methods that enable expansion of load immediate

Implement methods that enable expansion of load immediate
macro instruction (li) in the assembler.

We have identified three possible expansions depending on
the size of immediate operand:
  1) for 0 ≤ j ≤ 65535.
     li d,j =>
     ori d,$zero,j

  2) for −32768 ≤ j < 0.
     li d,j =>
     addiu d,$zero,j

  3) for any other value of j that is representable as a 32-bit integer.
     li d,j =>
     lui d,hi16(j)
     ori d,d,lo16(j)

All of the above have been implemented in ths patch.

Contributer: Vladimir Medic

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165199 91177308-0d34-0410-b5e6-96231b3b80d8