[TOSA] fix legalization of tfl.quantize
Hi @rsuderman, Can you please review this PR ? Thank you!
legalization of tfl.quantize used to do:
  - multiply by scale in f32, add zeropoint in f32, cast to output type
this led to mismatches because tosa cast (fp to int) used rounding mode: "round to nearest, tie to even",
so rounding can be wrong when adding the zeropoint before rounding.

this patch changes legalization to do:
  - multiply by scale in f32, cast to i32, add zeropoint in i32, cast to output type
  - also, if zeropoint is 0, then skip the "cast to i32" and "add zeropoint"


Change-Id: Ibaacd24778d83dd2cf1b1d914b287e32f4d775d0