Rollup merge of #125266 - workingjubilee:stream-plastic-love, r=RalfJung,nikic

compiler: add simd_ctpop intrinsic

Fairly straightforward addition.

cc `@rust-lang/opsem` new (extremely boring) intrinsic

Author: matthiaskrgr

src/intrinsics/simd.rs

@@ -348,6 +348,7 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
         | sym::simd_bswap
         | sym::simd_bitreverse
         | sym::simd_ctlz
+        | sym::simd_ctpop
         | sym::simd_cttz => {
             intrinsic_args!(fx, args => (a); intrinsic);
 
@@ -367,6 +368,7 @@ pub(super) fn codegen_simd_intrinsic_call<'tcx>(
                 (ty::Uint(_) | ty::Int(_), sym::simd_bswap) => fx.bcx.ins().bswap(lane),
                 (ty::Uint(_) | ty::Int(_), sym::simd_bitreverse) => fx.bcx.ins().bitrev(lane),
                 (ty::Uint(_) | ty::Int(_), sym::simd_ctlz) => fx.bcx.ins().clz(lane),
+                (ty::Uint(_) | ty::Int(_), sym::simd_ctpop) => fx.bcx.ins().popcnt(lane),
                 (ty::Uint(_) | ty::Int(_), sym::simd_cttz) => fx.bcx.ins().ctz(lane),
 
                 _ => unreachable!(),