Mips assembler: Enable handling of nested expressions

This patch allows the Mips assembler to parse and emit nested 
expressions as instruction operands. It also extends the 
expansion of memory instructions when an offset is given as 
an expression. 

Contributer: Vladimir Medic



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179657 91177308-0d34-0410-b5e6-96231b3b80d8