Inefficient function prologue

Originally reported on Google Code with ID 74

```
Local variable poisoning in a function prologue does not use the fact that they are
adjacent in memory and recalculates shadow address for each variable.

With origins is becomes even worse: we call a runtime function (__msan_set_alloca_origin4)
for each local variable. It should be enough to do it once per function entry, or even
less (with a fast path).

$ cat 1.cc
int use(long&, long&, long&, long&, long&, long&, long&, long&);
void f() {
long a, b, c, d, e, f, g, h;
use(a, b, c, d, e, f, g, h);
}

$ ./build/bin/clang++ 1.cc -fsanitize=memory -O2 -S -o -
.text
.file "1.cc"
.globl _Z1fv
.align 16, 0x90
.type _Z1fv,@function
_Z1fv: # @_Z1fv
.cfi_startproc
# BB#0: # %entry
pushq %rbx
.Ltmp0:
.cfi_def_cfa_offset 16
subq $80, %rsp
.Ltmp1:
.cfi_def_cfa_offset 96
.Ltmp2:
.cfi_offset %rbx, -16
movabsq $-70368744177672, %r10 # imm = 0xFFFFBFFFFFFFFFF8
leaq 72(%rsp), %rdi
movq %rdi, %rcx
andq %r10, %rcx
movq $-1, (%rcx)
leaq 64(%rsp), %rsi
movq %rsi, %rcx
andq %r10, %rcx
movq $-1, (%rcx)
leaq 56(%rsp), %rdx
movq %rdx, %rcx
andq %r10, %rcx
movq $-1, (%rcx)
leaq 48(%rsp), %rcx
movq %rcx, %rax
andq %r10, %rax
movq $-1, (%rax)
leaq 40(%rsp), %r8
movq %r8, %rax
andq %r10, %rax
movq $-1, (%rax)
leaq 32(%rsp), %r9
movq %r9, %rax
andq %r10, %rax
movq $-1, (%rax)
leaq 24(%rsp), %r11
movq %r11, %rax
andq %r10, %rax
movq $-1, (%rax)
leaq 16(%rsp), %rbx
andq %rbx, %r10
movq $-1, (%r10)
movq __msan_param_tls@GOTTPOFF(%rip), %rax
movq $0, %fs:(%rax)
movq $0, %fs:8(%rax)
movq $0, %fs:16(%rax)
movq $0, %fs:24(%rax)
movq $0, %fs:32(%rax)
movq $0, %fs:40(%rax)
movq $0, %fs:48(%rax)
movq $0, %fs:56(%rax)
movq __msan_retval_tls@GOTTPOFF(%rip), %rax
movl $0, %fs:(%rax)
movq %rbx, 8(%rsp)
movq %r11, (%rsp)
callq _Z3useRlS_S_S_S_S_S_S_@PLT
addq $80, %rsp
popq %rbx
retq
.Ltmp3:
.size _Z1fv, .Ltmp3-_Z1fv
.cfi_endproc

.section .init_array.0,"aw",@init_array
.align 8
.quad __msan_init

.ident "clang version 3.6.0 (trunk 222541)"
.section ".note.GNU-stack","",@progbits
```

Reported by `eugenis@google.com` on 2014-11-24 11:15:11