[mips] Refine octeon instructions seq/seqi/sne/snei

This commit refines the pattern for the octeon seq/seqi/sne/snei instructions.
The target register is set to 0 or 1 according to the result of the comparison.
In C, this is something like

rd = (unsigned long)(rs == rt)

This commit adds a zext to bring the result to i64. With this change the
instruction is selected for this type of code. (gcc produces the same code for
the above C code.)

llvm-svn: 225968