remove miri _mm256_permute2f128 fallback implementation

it is no longer used (and was also incorrect)

Author: folkertdev

src/tools/miri/src/shims/x86/avx.rs

@@ -217,50 +217,6 @@ pub(super) trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
  )?;
  }
  }
- // Used to implement the _mm256_permute2f128_ps, _mm256_permute2f128_pd and
- // _mm256_permute2f128_si256 functions. Regardless of the suffix in the name
- // thay all can be considered to operate on vectors of 128-bit elements.
- // For each 128-bit element of `dest`, copies one from `left`, `right` or
- // zero, according to `imm`.
- "vperm2f128.ps.256" | "vperm2f128.pd.256" | "vperm2f128.si.256" => {
- let [left, right, imm] =
- this.check_shim_sig_lenient(abi, CanonAbi::C, link_name, args)?;
-
- assert_eq!(dest.layout, left.layout);
- assert_eq!(dest.layout, right.layout);
- assert_eq!(dest.layout.size.bits(), 256);
-
- // Transmute to `[u128; 2]` to process each 128-bit chunk independently.
- let u128x2_layout =
- this.layout_of(Ty::new_array(this.tcx.tcx, this.tcx.types.u128, 2))?;
- let left = left.transmute(u128x2_layout, this)?;
- let right = right.transmute(u128x2_layout, this)?;
- let dest = dest.transmute(u128x2_layout, this)?;
-
- let imm = this.read_scalar(imm)?.to_u8()?;
-
- for i in 0..2 {
- let dest = this.project_index(&dest, i)?;
-
- let imm = match i {
- 0 => imm & 0xF,
- 1 => imm >> 4,
- _ => unreachable!(),
- };
- if imm & 0b100 != 0 {
- this.write_scalar(Scalar::from_u128(0), &dest)?;
- } else {
- let src = match imm {
- 0b00 => this.project_index(&left, 0)?,
- 0b01 => this.project_index(&left, 1)?,
- 0b10 => this.project_index(&right, 0)?,
- 0b11 => this.project_index(&right, 1)?,
- _ => unreachable!(),
- };
- this.copy_op(&src, &dest)?;
- }
- }
- }
  // Used to implement the _mm_maskload_ps, _mm_maskload_pd, _mm256_maskload_ps
  // and _mm256_maskload_pd functions.
  // For the element `i`, if the high bit of the `i`-th element of `mask`