[X86] Avoid large constants for min_signed_value check

[RKSimon]

https://godbolt.org/z/hhK4h1Es1

bool cmp_min_signed_value_i32(uint32_t x) {
    return x == 0x80000000;
}
bool cmp_min_signed_value_i64(uint64_t x) {
    return x == 0x8000000000000000ULL;
}

cmp_min_signed_value_i32(unsigned int): # @cmp_min_signed_value_i32(unsigned int)
  cmpl $-2147483648, %edi # imm = 0x80000000
  sete %al
  retq
cmp_min_signed_value_i64(unsigned long): # @cmp_min_signed_value_i64(unsigned long)
  movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000
  cmpq %rax, %rdi
  sete %al
  retq

For cases where we're testing for i32/i64 min_signed_value values, we could just negate the result and copy the overflow flag:

cmp_min_signed_value_i32:
  negl %edi
  seto %al
  retq
cmp_min_signed_value_i64:
  negq %rdi
  seto %al
  retq

i8/i16 might require sign-extension to avoid stalls, which might not be worth it:

cmp_min_signed_value_i8:
  movsx %dl, %edi
  negb %dl
  seto %al
  retq
cmp_min_signed_value_i16:
  movsx %di, %edi
  negw %di
  seto %al
  retq

[llvmbot]

@llvm/issue-subscribers-backend-x86

https://godbolt.org/z/hhK4h1Es1 ```c bool cmp_min_signed_value_i32(uint32_t x) { return x == 0x80000000; } bool cmp_min_signed_value_i64(uint64_t x) { return x == 0x8000000000000000ULL; } ``` ```asm cmp_min_signed_value_i32(unsigned int): # @cmp_min_signed_value_i32(unsigned int) cmpl $-2147483648, %edi # imm = 0x80000000 sete %al retq cmp_min_signed_value_i64(unsigned long): # @cmp_min_signed_value_i64(unsigned long) movabsq $-9223372036854775808, %rax # imm = 0x8000000000000000 cmpq %rax, %rdi sete %al retq ``` For cases where we're testing for i32/i64 min_signed_value values, we could just negate the result and copy the overflow flag: ```asm cmp_min_signed_value_i32: negl %edi seto %al retq cmp_min_signed_value_i64: negq %rdi seto %al retq ``` i8/i16 might require sign-extension to avoid stalls, which might not be worth it: ```asm cmp_min_signed_value_i8: movsx %dl, %edi negl %edi seto %al retq cmp_min_signed_value_i16: movsx %di, %edi negl %edi seto %al retq ```