WebAssembly: simply assert on SNaN and NaNs with payloads

[jfb]

Summary

convertToHexString doesn't represent them correctly at this point in time. This is a follow-up to sunfish's suggestion in D11914.

lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

for (const MachineOperand &MO : MI->uses())
     } break;
     case MachineOperand::MO_Immediate: {
       OS << ' ' << MO.getImm();
     } break;
     case MachineOperand::MO_FPImmediate: {
       static const size_t BufBytes = 128;
       char buf[BufBytes];
       APFloat FP = MO.getFPImm()->getValueAPF();
-      const APFloat CanonicalNaN = APFloat::getQNaN(FP.getSemantics());
-      if (FP.isNaN() && !FP.bitwiseIsEqual(CanonicalNaN))
-        // WebAssembly only has NaNs that are positive, quiet, without payload.
-        FP = CanonicalNaN;
+      if (FP.isNaN())
+        assert((FP.bitwiseIsEqual(APFloat::getQNaN(FP.getSemantics())) ||
+                FP.bitwiseIsEqual(
+                    APFloat::getQNaN(FP.getSemantics(), /*Negative=*/true))) &&
+               "convertToHexString handles neither SNaN nor NaN payloads");
       // Use C99's hexadecimal floating-point representation.
       auto Written =
           FP.convertToHexString(buf, /*hexDigits=*/0, /*upperCase=*/false,
                                 APFloat::rmNearestTiesToEven);
       assert(Written != 0);
       assert(Written < BufBytes);
       OS << ' ' << buf;
     } break;

test/CodeGen/WebAssembly/immediates.ll

 ; CHECK-LABEL: nan_f32:
 ; CHECK-NEXT: (setlocal @0 (immediate nan))
 ; CHECK-NEXT: (return @0)
 define float @nan_f32() {
   ret float 0x7FF8000000000000
 }
 
 ; CHECK-LABEL: negnan_f32:
-; CHECK-NEXT: (setlocal @0 (immediate nan))
+; CHECK-NEXT: (setlocal @0 (immediate -nan))
 ; CHECK-NEXT: (return @0)
 define float @negnan_f32() {
   ret float 0xFFF8000000000000
 }
 
 ; CHECK-LABEL: inf_f32:
 ; CHECK-NEXT: (setlocal @0 (immediate infinity))
 ; CHECK-NEXT: (return @0)

 ; CHECK-LABEL: nan_f64:
 ; CHECK-NEXT: (setlocal @0 (immediate nan))
 ; CHECK-NEXT: (return @0)
 define double @nan_f64() {
   ret double 0x7FF8000000000000
 }
 
 ; CHECK-LABEL: negnan_f64:
-; CHECK-NEXT: (setlocal @0 (immediate nan))
+; CHECK-NEXT: (setlocal @0 (immediate -nan))
 ; CHECK-NEXT: (return @0)
 define double @negnan_f64() {
   ret double 0xFFF8000000000000
 }
 
 ; CHECK-LABEL: inf_f64:
 ; CHECK-NEXT: (setlocal @0 (immediate infinity))
 ; CHECK-NEXT: (return @0)