[PATCH][rs6000][PR92379] fix UB shift of 64-bit type by 64 bits

[Aaron Sawdey via Gcc-patches]

This is a fix for PR92379. Passes regstrap on ppc64le. Pre-approved by 
Segher, committing after posting.

2020-03-13  Aaron Sawdey <acsaw...@linux.ibm.com>
    PR target/92379
    * config/rs6000/rs6000.c (num_insns_constant_multi) Don't shift a
    64-bit value by 64 bits (UB).

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 24598aff663..5798f924472 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -5612,7 +5612,10 @@ num_insns_constant_multi (HOST_WIDE_INT value, 
machine_mode mode)
       && rs6000_is_valid_and_mask (GEN_INT (low), DImode))
     insns = 2;
       total += insns;
-      value >>= BITS_PER_WORD;
+      /* If BITS_PER_WORD is the number of bits in HOST_WIDE_INT, doing
+     it all at once would be UB. */
+      value >>= (BITS_PER_WORD - 1);
+      value >>= 1;
     }
   return total;
 }