tcg: Add INDEX_op_dupm_vec

tcg: Add INDEX_op_dupm_vec

Allow the backend to expand dup from memory directly, instead of
forcing the value into a temp first.  This is especially important
if integer/vector register moves do not exist.

Note that officially tcg_out_dupm_vec is allowed to fail.
If it did, we could fix this up relatively easily:

  VECE == 32/64:
    Load the value into a vector register, then dup.
    Both of these must work.

  VECE == 8/16:
    If the value happens to be at an offset such that an aligned
    load would place the desired value in the least significant
    end of the register, go ahead and load w/garbage in high bits.

    Load the value w/INDEX_op_ld{8,16}_i32.
    Attempt a move directly to vector reg, which may fail.
    Store the value into the backing store for OTS.
    Load the value into the vector reg w/TCG_TYPE_I32, which must work.
    Duplicate from the vector reg into itself, which must work.

All of which is well and good, except that all supported
hosts can support dupm for all vece, so all of the failure
paths would be dead code and untestable.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>