[Bug target/115755] mulx (with -mbmi2) does not show up with constant multiply

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115755

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Hongtao Liu from comment #1)
> mulx doesn't support imm operand, a register is still needed to put 123.
> mulq is used func/func1 should be ok.

Right, but mulx does not set the flags so it should be better to move around
and such.

clang produces mulx for this.
func:                                   # @func
        .cfi_startproc
# %bb.0:
        movl    $123, %eax
        movq    %rdi, %rdx
        mulxq   %rax, %rax, %rax
        retq

func1:                                  # @func1
        .cfi_startproc
# %bb.0:
        movl    $123, %eax
        movq    %rdi, %rdx
        mulxq   %rax, %rax, %rdx
        retq