[Bug target/69946] [6 Regression] Invalid ppc64 assembly emitted starting with r226005

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69946

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 37803
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37803&action=edit
gcc6-pr69946.patch

What about this untested patch?  Ideas for better name of the new helper
function?  Seems that both rs6000_is_valid_shift_mask and
rs6000_insn_for_shift_mask were doing pretty much the same thing in their first
halves (preparation), except that for insert we assume that the shift count is
always CONST_INT.  So, this patch moves all that preparation into the helper,
and uses it in all the 4 functions.