[AArch64] Mark the read of the threadpointer has having unmodeled sid…

…e effects to prevent misoptimization in programs where the executing thread may change across a function call.

In principle, this change could be made more precise to enable some
optimizations of this operation, including removal of redundant reads
when there is no intervening function call.  But it is not clear what
attributes currently exist that could be attached to this operation to
model its behavior more precisely.

llvm/lib/Target/AArch64/AArch64InstrInfo.td

@@ -1513,8 +1513,10 @@ def : Pat<(AArch64mrs imm:$id),
 
 // The thread pointer (on Linux, at least, where this has been implemented) is
 // TPIDR_EL0.
+let hasSideEffects = 1 in {
 def MOVbaseTLS : Pseudo<(outs GPR64:$dst), (ins),
                        [(set GPR64:$dst, AArch64threadpointer)]>, Sched<[WriteSys]>;
+}
 
 // This gets lowered into a 24-byte instruction sequence
 let Defs = [ X9, X16, X17, NZCV ], Size = 24 in {