BaseTools ConvertMasmToNasm: Don't try to reconvert .nasm files
BaseTools ConvertMasmToNasm: Don't try to reconvert .nasm files

We now check to see if the destination .nasm file already exists. If
it does, then we don't try to convert the .asm to .nasm.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>
Reviewed-by: Liming Gao <6480311aeeb4b006862f6d13ebabddc03f51e507@intel.com>

MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2,MattDevo/edk2