patch to fix PR69847

[Vladimir Makarov]

   The following patch is for

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69847

   The patch implements an invariant inheritance and some hard reg 
assignment improvements in LRA.  Actually I tried several approaches to 
implement this.  This patch is what worked best (a compromise between 
complexity and performance results).

   The patch was successfully tested and bootstrapped on x86-64 and 
ppc64.  As the patch changes a complicated code (inheritance code) in 
LRA, there is non-zero probability of regressions but I will work to fix 
them if/when they are reported.

   Committed as rev. 238991.


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