[WebAssembly] Avoid `bit_cast` when printing f32 and f64 immediates

[sunfish]

Summary

Use APInt to convert a 32-bit or 64-bit immediate to an APFloat rather than bit_cast to a float or double to avoid going through host floating-point and potentially changing the bit pattern of NaNs.

[lbenes]

I can confirm that this patch fixes the failing immediates.ll test on x86 built by gcc 10.

llvm/lib/Target/WebAssembly/MCTargetDesc/WebAssemblyInstPrinter.cpp

if (Op.isReg()) {
     else
       O << "$drop";
     // Add a '=' suffix if this is a def.
     if (OpNo < MII.get(MI->getOpcode()).getNumDefs() || IsVariadicDef)
       O << '=';
   } else if (Op.isImm()) {
     O << Op.getImm();
   } else if (Op.isSFPImm()) {
-    O << ::toString(APFloat(bit_cast<float>(Op.getSFPImm())));
+    O << ::toString(APFloat(APFloat::IEEEsingle(), APInt(32, Op.getSFPImm())));
   } else if (Op.isDFPImm()) {
-    O << ::toString(APFloat(bit_cast<double>(Op.getDFPImm())));
+    O << ::toString(APFloat(APFloat::IEEEdouble(), APInt(64, Op.getDFPImm())));
   } else {
     assert(Op.isExpr() && "unknown operand kind in printOperand");
     // call_indirect instructions have a TYPEINDEX operand that we print
     // as a signature here, such that the assembler can recover this
     // information.
     auto SRE = static_cast<const MCSymbolRefExpr *>(Op.getExpr());
     if (SRE->getKind() == MCSymbolRefExpr::VK_WASM_TYPEINDEX) {
       auto &Sym = static_cast<const MCSymbolWasm &>(SRE->getSymbol());