Fix SQRT.S and RSQRT.S operand selection in the FPU translator

[yanm1103]

Problem

FpuTranslator picks the wrong operands for two COP1.S instructions:

• SQRT.S was emitted as ctx->f[fd] = FPU_SQRT_S(ctx->f[fs]);
• RSQRT.S was emitted as ctx->f[fd] = 1.0f / sqrtf(ctx->f[fs]);

R5900 rule

• SQRT.S fd, ft: the source operand is the ft field. The fs field is 0 in the encoding.
• RSQRT.S fd, fs, ft: the result is fs / sqrt(ft), not 1 / sqrt(fs).
• Both use the magnitude of ft, so a negative ft does not give NaN (this is what PCSX2's FPU.cpp does: SQRT_S and RSQRT_S apply sqrt(fabs(ft)) for negative ft, and RSQRT_S divides fs by it).

For example sqrt.s $f2, $f2 is the word 0x46020084 (fmt=S, ft=2, fs=0, fd=2, funct=4). It was translated to ctx->f[2] = FPU_SQRT_S(ctx->f[0]);, i.e. the square root of the wrong register.

Fix

case COP1_S_SQRT:  ctx->f[fd] = FPU_SQRT_S(fabsf(ctx->f[ft]));
case COP1_S_RSQRT: ctx->f[fd] = ctx->f[fs] / sqrtf(fabsf(ctx->f[ft]));

Scope is these two cases only. Zero and denormal handling and the FPU flag updates that PCSX2 models are not touched here.

Verification

• Found while recompiling a retail PS2 game. Its ELF contains the buggy pattern in 108 functions (sqrt.s) and 76 functions (rsqrt.s).
• I ran an original function and an independent reimplementation, both recompiled with PS2Recomp, on 300 random states. Without this fix the results disagreed in 300/300 runs (the sum of squares in $f2 was ignored). With the fix they matched 300/300.
• Added a ps2xTest case in code_generator_tests.cpp that checks the emitted code for sqrt.s $f2, $f2 (0x46020084) and for rsqrt.s $f1, $f3, $f4. I could not run the full CMake test suite locally; I compiled fpu_translator.cpp standalone and checked that it emits exactly the strings the test expects.

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