libc: Prevent FCSEL instruction from being used to avoid raising an unintended exception

[papparapa]

closes #24184

According to the comment, this line of the current sin function evaluates x + 0x1p120 and raises an INEXACT exception if x is not zero.
It also evaluates x / 0x1p120 and raises an UNDERFLOW exception if x is a non-normalized number.

// raise inexact if x != 0 and underflow if subnormal
if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);

This is the disassembly results of the sin function test built in #24184.

$ llvm-objdump -S -l -d ./zig-out/bin/sin
[omitted]
; /workspaces/zig/lib/compiler_rt/sin.zig:101
;             if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);
 1075300: f144011f     	cmp	x8, #0x100, lsl #12     // =0x100000
 1075304: 9e670121     	fmov	d1, x9
 1075308: 9e670142     	fmov	d2, x10
 107530c: 910023e8     	add	x8, sp, #0x8
 1075310: 1e610801     	fmul	d1, d0, d1
 1075314: 1e622802     	fadd	d2, d0, d2
 1075318: 1e623c21     	fcsel	d1, d1, d2, lo
[omitted]

FCSEL instruction selects one of the two registers according to the condition and copies its value.
The FCSEL instruction is preceded by the FMUL and FADD instructions, indicating that both x / 0x1p120 and x + 0x1p120 are evaluated.
It selects one or the other based on the result of ix < 0x00100000, but if a floating point exception occurs in the calculation of the one not selected, the exception is reflected in the final result.
In other words, it is effective in reducing branching, but both the FMUL and FADD instructions can have the side effect of raising a floating point exception, which may cause an exception that the programmer does not intend.

To prevent FCSEL instruction from being used here, this PR splits doNotOptimizeAway in two.
This change adds a branch instruction, but since this code path only passes if x is very small (|x| < 2^-26 in sin), I believe this change is acceptable.

In fact, after this PR, unintended exceptions no longer occur.

$ ./zig-out/bin/sin
$

Several other functions are similarly changed to prevent from raising unintended exceptions.

• sin
• sinf
• sincos
• sincosf
• tan
• tanf

[jacobly0]

Looks reasonable.