Don't hide alignment from 'memset'.

Test code:

#include <string.h>
char* mem_demo_1(char *j)
{ // *BAD* compiler cannot tell pointer alignment, must test
    memset(j, 0, 64);
    return j;
}
char* mem_demo_2(void)
{ // *GOOD* compiler can tell pointer alignment
    char * j = malloc(64);
    memset(j, 0, 64);
    return j;
}

Compiled with GCC 4.6.3, options -O3. Also tested with -O2, results are about the same.

mem_demo_1:
.LFB12:
    .cfi_startproc
    movq    %rdi, %rsi
    movl    $64, %edx
    testb   $1, %sil
    jne .L9
    testb   $2, %dil
    jne .L10
.L3:
    testb   $4, %dil
    jne .L11
.L4:
    movl    %edx, %ecx
    xorl    %eax, %eax
    shrl    $3, %ecx
    testb   $4, %dl
    rep stosq
    je  .L5
    movl    $0, (%rdi)
    addq    $4, %rdi
.L5:
    testb   $2, %dl
    je  .L6
    movw    $0, (%rdi)
    addq    $2, %rdi
.L6:
    andl    $1, %edx
    je  .L7
    movb    $0, (%rdi)
.L7:
    movq    %rsi, %rax
    ret
    .p2align 4,,10
    .p2align 3
.L9:
    leaq    1(%rsi), %rdi
    movb    $0, (%rsi)
    movb    $63, %dl
    testb   $2, %dil
    je  .L3
    .p2align 4,,10
    .p2align 3
.L10:
    movw    $0, (%rdi)
    addq    $2, %rdi
    subl    $2, %edx
    testb   $4, %dil
    je  .L4
    .p2align 4,,10
    .p2align 3
.L11:
    movl    $0, (%rdi)
    subl    $4, %edx
    addq    $4, %rdi
    jmp .L4
    .cfi_endproc
...

mem_demo_2:
.LFB13:
    .cfi_startproc
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    movl    $64, %edi
    call    malloc
    movq    %rax, %rdx
    movl    $8, %ecx
    xorl    %eax, %eax
    movq    %rdx, %rdi
    rep stosq
    movq    %rdx, %rax
    addq    $8, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc