This patch implements vector operations on MIPS32 using VariableVecOn32 method (on the lines of Variable64On32).
Vector operations are scalarized prior to lowering. Each vector variable is split into 4 containers to hold a variable of vector type.
For MIPS32, four GP/FP registers are used to hold a vector variable. Arguments are passed in GP registers irrespective of the type of the vector variable.

Lit test vector-mips.ll has been added to test this implementation.

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2380023002 .

Patch from Jaydeep Patil <jaydeep.patil@imgtec.com>.