rs6000: Properly handle LE index munging in vec_shr (PR94710)

The PR shows the compiler crashing with -mvsx -mlittle -O0.  This turns
out to be caused by a failure to make of the higher bits in an index
endian conversion.

2020-04-24  Segher Boessenkool  <segher@kernel.crashing.org>

	PR target/94710
	* config/rs6000/vector.md (vec_shr_<mode> for VEC_L): Correct little
	endian byteshift_val calculation.

gcc/ChangeLog

+2020-04-24  Segher Boessenkool  <segher@kernel.crashing.org>
+
+	PR target/94710
+	* config/rs6000/vector.md (vec_shr_<mode> for VEC_L): Correct little
+	endian byteshift_val calculation.
+
 2020-04-24  Andrew Stubbs  <ams@codesourcery.com>
 
 	* config/gcn/gcn.md (*mov<mode>_insn): Only split post-reload.

gcc/config/rs6000/vector.md

@@ -1426,7 +1426,8 @@
   emit_move_insn (zero_reg, CONST0_RTX (<MODE>mode));
   if (!BYTES_BIG_ENDIAN)
     {
-      byteshift_val = 16 - byteshift_val;
+      /* Note, byteshift_val can be 0!  */
+      byteshift_val = -byteshift_val & 15;
       op1 = zero_reg;
       op2 = operands[1];
     }