Fix issue 20052 - SIMD 32 bytes segfault

[SSoulaimane]

issue 20052.

When doing runtime stack alignment, the stack frame must be saved otherwise the return address will be lost.

For example:
the following function

struct y { __vector(long[4]) v; }
y get()
{
    y a;
    return a;
}

generated the following instructions:

and    rsp,0xffffffffffffffe0
xor    eax,eax
mov    QWORD PTR [rdi],rax
mov    QWORD PTR [rdi+0x8],rax
mov    QWORD PTR [rdi+0x10],rax
mov    QWORD PTR [rdi+0x18],rax
mov    rax,rdi
ret

Notice there is no RBP. When the return procedure is reached, the ret instruction pops the return address from the stack which has been altered earlier by the alignment instruction, which crashes the program.

This patch does two things: 1) make sure BP is stored, and a proper leave procedure is used whenever there is runtime stack alignent. 2) omit runtime alignment when all symbols are dead.