The Arm MVE VDUP implementation would like to be able to emit code to
duplicate a byte or halfword value into an i32.  We have code to do
this already in tcg-op-gvec.c, so all we need to do is make the
functions global.

For consistency with other functions made available to the frontends:
 * we rename to tcg_gen_dup_*
 * we expose both the _i32 and _i64 forms
 * we provide the #define for a _tl form

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210617121628.20116-10-peter.maydell@linaro.org