* Ensure that a source position is already specified in generated code before
  prologue is assembled.
* Ensure source position is set for instructions before their gaps are assembled
  (this fixes missing source position information at the beginning of deferred
  code).
* Don't output source position information for gap moves that are
  redundant. This led to extraneous, confusing source positions for constants
  that did not end up producing any code.
* Output source position information that is usable in turbolizer when --trace-turbo
  is specified.

LOG=N

Review-Url: https://codereview.chromium.org/2599433002
Cr-Commit-Position: refs/heads/master@{#41914}