[clang] mips4 instructions generated when compiling for mips2

[Sirius902]

The Problem

When using the __atomic_fetch_min and __atomic_fetch_max compiler builtins it is possible to generate instructions introduced in mips4 despite compiling for mips2. Following the code below compiling with -mips2, using __atomic_fetch_min generates a movz instruction while __atomic_fetch_max generates a movn instruction.

Here is C code that demonstrates this problem along with a Godbolt link demonstrating this miscompilation.

int min(int* x, int y) {
    return __atomic_fetch_min(x, y, __ATOMIC_SEQ_CST);
}

int max(int* x, int y) {
    return __atomic_fetch_max(x, y, __ATOMIC_SEQ_CST);
}

Expected Behavior

I would expect that instructions like movz and movn would not be emitted when compiling for -mips2 since these instructions were introduced in MIPS IV, they are not present in the MIPS II standard.

[llvmbot]

@llvm/issue-subscribers-backend-mips

Author: None (Sirius902)

## The Problem

When using the __atomic_fetch_min and __atomic_fetch_max compiler builtins it is possible to generate instructions introduced in mips4 despite compiling for mips2. Following the code below compiling with -mips2, using __atomic_fetch_min generates a movz instruction while __atomic_fetch_max generates a movn instruction.

Here is C code that demonstrates this problem along with a Godbolt link demonstrating this miscompilation.

int min(int* x, int y) {
    return __atomic_fetch_min(x, y, __ATOMIC_SEQ_CST);
}

int max(int* x, int y) {
    return __atomic_fetch_max(x, y, __ATOMIC_SEQ_CST);
}

Expected Behavior

I would expect that instructions like movz and movn would not be emitted when compiling for -mips2 since these instructions were introduced in MIPS IV, they are not present in the MIPS II standard.

[brad0]

cc @yingopq @wzssyqa

[brad0]

@Sirius902 Can you please test the PR?

[brad0]

/cherry-pick b7f68cb