IR: Fold ClampToZero into the 31-bit packs

Vec4ClampToZero and Vec2ClampToZero only ever fed Vec4Pack31To8 and
Vec2Pack31To16, for vi2uc and vi2us. The packs now clamp negative lanes
to zero themselves, which saves an op and a vector temp, and lets x64
clamp with PACKUSWB's saturation after an arithmetic shift.

While at it, RISC-V compiles Vec2Unpack16To31, Vec2Pack31To16 and
Vec4Pack32To8, and LoongArch Vec2Unpack16To31, Vec2Pack31To16 and the
non-LSX Vec4Pack32To8, all of which went to the IR interpreter.
LoongArch's Vec2Pack32To16 and Vec2Unpack16To32 now take their scalar
path with LSX too, instead of falling back.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/MIPS/IR/IRNativeCommon.cpp

@@ -422,11 +422,6 @@ void IRNativeBackend::CompileIRInst(IRInst inst) {
 		CompIR_VecPack(inst);
 		break;
 
-	case IROp::Vec4ClampToZero:
-	case IROp::Vec2ClampToZero:
-		CompIR_VecClamp(inst);
-		break;
-
 	case IROp::FSin:
 	case IROp::FCos:
 	case IROp::FRSqrt: