[NFC][AArch64] Explicitly define undefined bits for instructions (#122129)

Explicitly define bits for the following instructions:
  * Bit 21 for stltxr/ldatxr (defaults to zero) wasn't defined.
  * Bits 16-20 for ldaxpx/ldxpx (unpredictable) weren't defined.
  * Bits 10-14 for smulh/umulh (unpredictable) weren't defined.

Author: jthackray

llvm/lib/Target/AArch64/AArch64InstrFormats.td

@@ -2770,6 +2770,8 @@ class MulHi<bits<3> opc, string asm, SDNode OpNode>
   let Inst{23-21} = opc;
   let Inst{20-16} = Rm;
   let Inst{15}    = 0;
+  let Inst{14-10} = 0b11111;
+  let Unpredictable{14-10} = 0b11111;
   let Inst{9-5}   = Rn;
   let Inst{4-0}   = Rd;
 
@@ -4922,6 +4924,8 @@ class LoadExclusivePair<bits<2> sz, bit o2, bit L, bit o1, bit o0,
   bits<5> Rt;
   bits<5> Rt2;
   bits<5> Rn;
+  let Inst{20-16} = 0b11111;
+  let Unpredictable{20-16} = 0b11111;
   let Inst{14-10} = Rt2;
   let Inst{9-5} = Rn;
   let Inst{4-0} = Rt;
@@ -4937,6 +4941,7 @@ class BaseLoadStoreExclusiveLSUI<bits<2> sz, bit L, bit o0,
   let Inst{31-30} = sz;
   let Inst{29-23} = 0b0010010;
   let Inst{22}    = L;
+  let Inst{21}    = 0b0;
   let Inst{15}    = o0;
 }