Emit all directives except for ".cprestore" during asm printing rather than emit

them as machine instructions. Directives ".set noat" and ".set at" are now
emitted only at the beginning and end of a function except in the case where
they are emitted to enclose .cpload with an immediate operand that doesn't fit
in 16-bit field or unaligned load/stores.

Also, make the following changes:
- Remove function isUnalignedLoadStore and use a switch-case statement to
  determine whether an instruction is an unaligned load or store.

- Define helper function CreateMCInst which generates an instance of an MCInst
  from an opcode and a list of operands.

llvm-svn: 153552