This patch ensures consistency in the construction of FP_ROUND nodes
such that they always use ISD::TargetConstant instead of ISD::Constant.
This additionally fixes a bug in the AArch64 SVE backend where patterns
were matching against TargetConstant nodes and sometimes failing when
passed a Constant node.
Is there any real reason that FP_ROUND_MERGE_PASSTHRU even needs this flag? Or is just because LowerToPredicatedOp doesn't know to drop it?