JitArm64_LoadStore: Optimize zero stores in stX

The value being stored must be loaded into a register. In the case of an
immediate value, this means it must be materialized. The value is
eventually byteswapped before performing the store.

This can be simplified for the value 0 for two reasons:
- ARM64 has a dedicated zero register, so does not need to be
  materialized.
- Byteswapping zero is still zero, so we can skip this step.

We could skip byteswapping for other values by immediately materializing
the byteswapped value in a register, but the benefits are not so clear
there (if the value needs to be materialized anyway, it is better to do
it up front).

Before:
0x5280001b   mov    w27, #0x0                 ; =0
0xb9404fba   ldr    w26, [x29, #0x4c]
0x12881862   mov    w2, #-0x40c4              ; =-16580
0x0b020342   add    w2, w26, w2
0x5ac00b61   rev    w1, w27
0xb8226b81   str    w1, [x28, x2]

After:
0xb9404fbb   ldr    w27, [x29, #0x4c]
0x12881862   mov    w2, #-0x40c4              ; =-16580
0x0b020362   add    w2, w27, w2
0xb8226b9f   str    wzr, [x28, x2]

Source/Core/Core/PowerPC/JitArm64/JitArm64_LoadStore.cpp

@@ -181,7 +181,8 @@ void JitArm64::SafeStoreFromReg(s32 dest, u32 value, s32 regOffset, u32 flags, s
   if (!jo.fastmem)
     gpr.Lock(ARM64Reg::W0);
 
-  ARM64Reg RS = gpr.R(value);
+  // Don't materialize zero.
+  ARM64Reg RS = gpr.IsImm(value, 0) ? ARM64Reg::WZR : gpr.R(value);
 
   ARM64Reg reg_dest = ARM64Reg::INVALID_REG;
   ARM64Reg reg_off = ARM64Reg::INVALID_REG;

Source/Core/Core/PowerPC/JitArm64/Jit_Util.cpp

@@ -257,6 +257,12 @@ void ByteswapAfterLoad(ARM64XEmitter* emit, ARM64FloatEmitter* float_emit, ARM64
 ARM64Reg ByteswapBeforeStore(ARM64XEmitter* emit, ARM64FloatEmitter* float_emit, ARM64Reg tmp_reg,
                              ARM64Reg src_reg, u32 flags, bool want_reversed)
 {
+  // Byteswapping zero is still zero.
+  // We'd typically expect a writable register to be passed in, but recognize
+  // WZR for optimization purposes.
+  if ((flags & BackPatchInfo::FLAG_FLOAT) == 0 && src_reg == ARM64Reg::WZR)
+    return ARM64Reg::WZR;
+
   ARM64Reg dst_reg = src_reg;
 
   if (want_reversed == !(flags & BackPatchInfo::FLAG_REVERSE))