[X86] Correct the memory operand for the FLD emitted in FP_TO_INTHelper for 32-bit...

[X86] Correct the memory operand for the FLD emitted in FP_TO_INTHelper for 32-bit SSE targets.

We were using DstTy, but that represents the integer type we are converting to which is i64 in this
case. The FLD is part of an intermediate step to get from the SSE registers to the x87 registers.
If the floating point type is f32, the memory operand should reflect a 4 byte access not an 8 byte
access. The store we used to get from SSE to the stack is using the corect size.

While there, consistenly use TheVT in place of Op.getOperand(0).getValueType() throughout the function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353745 91177308-0d34-0410-b5e6-96231b3b80d8