[libc][NFC] Add a few casts to suppress loss of precision warnings

[sivachandra]

libc/utils/FPUtil/x86_64/FEnv.h

295–296

To make it absolutely clear, make it uint16_t(0x3).

[michaelrj]

clarify the type for the constant being shifted

libc/utils/FPUtil/x86_64/FEnv.h

case FE_UPWARD:
     break;
   case FE_TOWARDZERO:
     bitValue = internal::RoundingControlValue::TowardZero;
     break;
   default:
     return 1; // To indicate failure
   }
 
-  uint16_t x87Value = bitValue << internal::X87RoundingControlBitPosition;
+  uint16_t x87Value = static_cast<uint16_t>(
+      bitValue << internal::X87RoundingControlBitPosition);
   uint16_t x87Control = internal::getX87ControlWord();
-  x87Control =
-      (x87Control & ~(0x3 << internal::X87RoundingControlBitPosition)) |
-      x87Value;
+  x87Control = static_cast<uint16_t>(
+      (x87Control &
+       ~(uint16_t(0x3) << internal::X87RoundingControlBitPosition)) |
+      x87Value);
   internal::writeX87ControlWord(x87Control);
 
   uint32_t mxcsrValue = bitValue << internal::MXCSRRoundingControlBitPosition;
   uint32_t mxcsrControl = internal::getMXCSR();
   mxcsrControl =
       (mxcsrControl & ~(0x3 << internal::MXCSRRoundingControlBitPosition)) |
       mxcsrValue;
   internal::writeMXCSR(mxcsrControl);
 
   return 0;
 }
 
 } // namespace fputil
 } // namespace __llvm_libc
 
 #endif // LLVM_LIBC_UTILS_FPUTIL_X86_64_FENV_H