[X86] Implement smarter instruction lowering for FP_TO_UINT from f32/f64 to i32/i64...

[X86] Implement smarter instruction lowering for FP_TO_UINT from f32/f64 to i32/i64 and vXf32/vXf64 to vXi32 for SSE2 and AVX2 by using the exact semantic of the CVTTPS2SI instruction.

We know that "CVTTPS2SI" returns 0x80000000 for out of range inputs (and for FP_TO_UINT, negative float values are undefined). We can use this to make unsigned conversions from vXf32 to vXi32 more efficient, particularly on targets without blend using the following logic:

small := CVTTPS2SI(x);
fp_to_ui(x) := small | (CVTTPS2SI(x - 2^31) & ARITHMETIC_RIGHT_SHIFT(small, 31))

Even on targets where "PBLENDVPS"/"PBLENDVB" exists, it is often a latency 2, low throughput instruction so this logic is applied there too (in particular for AVX2 also). It furthermore gets rid of one high latency floating point comparison in the previous lowering.

@TomHender checked the correctness of this for all possible floats between -1 and 2^32 (both ends excluded).

Original Patch by @TomHender (Tom Hender)

Differential Revision: https://reviews.llvm.org/D89697