[MIR-Canon] Adding ISA-Agnostic COPY Folding.

Transforms the following:

    %vreg1234:gpr32 = COPY %42
    %vreg1235:gpr32 = COPY %vreg1234
    %vreg1236:gpr32 = COPY %vreg1235
    $w0 = COPY %vreg1236

into:

    $w0 = COPY %42

Assuming %42 is also a gpr32

llvm-svn: 330113