MIPS64: Fix tests for instructions LDPC and LWUPC.

In tests r6_ldpc and r6_lwupc are accidentally removed instructions
which should be actually tested - LDPC and LWUPC.
(See 822be9b2 "Normalize casing of
hexadecimal digits".)
This CL fixes this error in tests.

TEST=cctest/test-assembler-mips64/r6_ldpc,
     cctest/test-assembler-mips64/r6_lwupc
BUG=

Change-Id: I34b8da2c6b97c0d85bde4f1cd3b3c704a9d05208
Reviewed-on: https://chromium-review.googlesource.com/845623Reviewed-by: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#50313}

test/cctest/test-assembler-mips64.cc

@@ -5388,6 +5388,7 @@ uint64_t run_lwupc(int offset) {
     __ dd(addiu_new);
   }
 
+  __ lwupc(t8, offset);  // offset 0; 0xEF080000 (t8 register)
   __ mov(v0, t8);
 
   // 256k instructions; 2^8k
@@ -5957,6 +5958,7 @@ uint64_t run_ldpc(int offset) {
     __ dd(addiu_new);
   }
 
+  __ ldpc(t8, offset);  // offset 0; 0xEF080000 (t8 register)
   __ mov(v0, t8);
 
   // 256k instructions; 2 * 2^7k = 2^8k