[gcc] Fix some more gcc-only warnings

Bug: chromium:1307180
Change-Id: Ia279086ec3df9d64bd6a0255ebe27d9e583014c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732937Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81433}

src/diagnostics/ia32/disasm-ia32.cc

@@ -545,7 +545,7 @@ int DisassemblerIA32::F7Instruction(byte* data) {
   byte modrm = *++data;
   int mod, regop, rm;
   get_modrm(modrm, &mod, &regop, &rm);
-  const char* mnem = nullptr;
+  const char* mnem = "";
   switch (regop) {
     case 0:
       mnem = "test";
@@ -587,7 +587,7 @@ int DisassemblerIA32::D1D3C1Instruction(byte* data) {
   int mod, regop, rm;
   get_modrm(modrm, &mod, &regop, &rm);
   int imm8 = -1;
-  const char* mnem = nullptr;
+  const char* mnem = "";
   switch (regop) {
     case kROL:
       mnem = "rol";
@@ -2075,7 +2075,7 @@ int DisassemblerIA32::InstructionDecode(v8::base::Vector<char> out_buffer,
         data++;
         int mod, regop, rm;
         get_modrm(*data, &mod, &regop, &rm);
-        const char* mnem = nullptr;
+        const char* mnem = "";
         switch (regop) {
           case esi:
             mnem = "push";
@@ -2122,7 +2122,7 @@ int DisassemblerIA32::InstructionDecode(v8::base::Vector<char> out_buffer,
         data++;
         int mod, regop, rm;
         get_modrm(*data, &mod, &regop, &rm);
-        const char* mnem = nullptr;
+        const char* mnem = "";
         switch (regop) {
           case 5:
             mnem = "subb";

src/objects/js-temporal-objects.cc

@@ -11561,7 +11561,7 @@ Handle<BigInt> RoundNumberToIncrement(Isolate* isolate, Handle<BigInt> x,
   // 1. Let quotient be x / increment.
   bool is_negative;
   // 2. If quotient < 0, then
-  if (x->IsNegative() != increment < 0) {
+  if (x->IsNegative() != (increment < 0)) {
     // a. Let isNegative be true.
     is_negative = true;
     // b. Set quotient to -quotient.