[PATCH] Make sure to set trunc store action to Expand for all X86 targets.

[Luqman Aden]

When compiling without SSE2, isTruncStoreLegal(F64, F32) would return Legal, whereas with SSE2 it would return Expand. And since the Target doesn't seem to actually handle a truncstore for double -> float, it would just output a store of a full double in the space for a float hence overwriting other bits on the stack.

```
define void @bar() unnamed_addr {
entry-block:
  %a = alloca double
  %b = alloca float

  store double 3.140000e+00, double* %a
  %0 = load double* %a

  %1 = fptrunc double %0 to float
  store float %1, float* %b

  ret void
}
```

Without SSE2:
```
bar:                                    # @bar
    .cfi_startproc
# BB#0:                                 # %entry-block
    subl    $20, %esp
.Ltmp0:
    .cfi_def_cfa_offset 24
    movl    $1074339512, 12(%esp)   # imm = 0x40091EB8
    movl    $1374389535, 8(%esp)    # imm = 0x51EB851F                 0x40091EB851EB851F (double) 3.14
    movl    $1074339512, 8(%esp)    # imm = 0x40091EB8
    movl    $1374389535, 4(%esp)    # imm = 0x51EB851F
    addl    $20, %esp
    retl
```

With SSE2:
```
bar:                                    # @bar
    .cfi_startproc
# BB#0:                                 # %entry-block
    subl    $20, %esp
.Ltmp0:
    .cfi_def_cfa_offset 24
    movl    $1074339512, 12(%esp)   # imm = 0x40091EB8
    movl    $1374389535, 8(%esp)    # imm = 0x51EB851F            0x40091EB851EB851F (double) 3.14
    movl    $1078523331, 4(%esp)    # imm = 0x4048F5C3           0x4048F5C3 (float) 3.14
    addl    $20, %esp
    retl
```

http://reviews.llvm.org/D5125

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/dont-trunc-store-double-to-float.ll
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