Revert "using the original sourcemap as the base"
Revert "using the original sourcemap as the base"

This reverts commit ad18689d926d25c7a25b95c630c2ad05b7b5f5b5.

Reason for revert: introduce issue #882

Currently, generated sourcemap contains copy of all existing mappings and adds new mappings from uglified code to original one.
However, previous mapping are no longer valid and shouldn't be added.

davidbau/UglifyJS2,dabretin/UglifyJS2,avdg/UglifyJS2,rvanvelzen/UglifyJS2