Fix the invalid opcode for Mips branch instructions in the assembler

For mips a branch an 18-bit signed offset (the 16-bit 
offset field shifted left 2 bits) is added to the 
address of the instruction following the branch 
(not the branch itself), in the branch delay slot, 
to form a PC-relative effective target address. 

Previously, the code generator did not perform the 
shift of the immediate branch offset which resulted 
in wrong instruction opcode. This patch fixes the issue.

Contributor: Vladimir Medic
llvm-svn: 177687