Additional -Wnonnull errors happen with GCC 11

On behalf of Dan Lutker <lutkerd@amazon.com>

JDK-8271515 introduced more issues with xmm0 with GCC 11

Error :

gmake[2]: *** Waiting for unfinished jobs....
 
ERROR: Build failed for target 'images' in configuration 'linux-x86_64-server-release' (exit code 2)
Stopping sjavac server
 
=== Output from failing command(s) repeated here ===
* For target hotspot_variant-server_libjvm_objs_assembler_x86.o:
…/src/hotspot/cpu/x86/assembler_x86.cpp: In member function 'void Assembler::evprord(XMMRegister, KRegister, XMMRegister, int, bool, int)':
…/src/hotspot/cpu/x86/assembler_x86.cpp:11112:38: error: 'this' pointer is null [-Werror=nonnull]
11112 | int encode = vex_prefix_and_encode(xmm0->encoding(), dst->encoding(), src->encoding(), VEX_SIMD_66, VEX_OPCODE_0F, &attributes);
   | ^~~~
In file included from …/src/hotspot/share/asm/register.hpp:59,
         from …/src/hotspot/share/asm/assembler.hpp:29,
         from …/src/hotspot/cpu/x86/assembler_x86.cpp:26:
…/src/hotspot/cpu/x86/register_x86.hpp:160:9: note: in a call to non-static member function 'int XMMRegisterImpl::encoding() const'
 160 | int encoding() const { assert(is_valid(), "invalid register (%d)", (int)(intptr_t)this ); return (intptr_t)this; }
   | ^~~~~~~~
…/hotspot/cpu/x86/assembler_x86.cpp: In member function 'void Assembler::evprorq(XMMRegister, KRegister, XMMRegister, int, bool, int)':
…/src/hotspot/cpu/x86/assembler_x86.cpp:11124:38: error: 'this' pointer is null [-Werror=nonnull]
11124 | int encode = vex_prefix_and_encode(xmm0->encoding(), dst->encoding(), src->encoding(), VEX_SIMD_66, VEX_OPCODE_0F, &attributes);
   | ^~~~
In file included from …/src/hotspot/share/asm/register.hpp:59,
  ... (rest of output omitted)