https://zig.godbolt.org/z/36WWT1Kes

define <4 x i32> @doit() {
  %res = call <4 x i32> @llvm.fshl.v4i32(<4 x i32> <i32 0, i32 1, i32 2, i32 3>, <4 x i32> <i32 4, i32 5, i32 6, i32 7>, <4 x i32> <i32 8, i32 9, i32 10, i32 11>)
  ret <4 x i32> %res
}

llc -matr=+avx2

  vmovaps .LCPI0_0(%rip), %xmm0 # xmm0 = [0,512,2048,6144]
  retq

llc -mattr=+avx512vl,+avx512vbmi2

doit: # @doit
  vpmovsxbd .LCPI0_3(%rip), %xmm1 # xmm1 = [4,5,6,7]
  vpmovsxbd .LCPI0_4(%rip), %xmm0 # xmm0 = [0,1,2,3]
  vpshldvd .LCPI0_2(%rip), %xmm1, %xmm0
  retq

AVX2 manages to constant fold fshl/fshr only because we expand to regular shifts, AVX512VBMI2 targets have legal funnel shift instructions and have no opportunity to constant fold.

FoldConstantArithmetic only attempts to constant fold unary/binary scalars, but it shouldn't be difficult to handle trinary scalar integers as well for FSHL/R.