tcg: Add muluh and mulsh opcodes

Use them in places where mulu2 and muls2 are used.
Optimize mulx2 with dead low part to mulxh.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-mips: Implement mulsh, muluh

With the optimization in tcg_liveness_analysis,
we can avoid the MFLO when it is unused.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-ppc64: Implement muluh, mulsh

Using these instead of mulu2 and muls2 lets us avoid having to argument
overlap analysis in the backend.  Normal register allocation will DTRT.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg: Constant fold div, rem

Signed-off-by: Richard Henderson <rth@twiddle.net>