[llvm] Always use TargetConstant for FP_ROUND ISD Nodes

[llvm] Always use TargetConstant for FP_ROUND ISD Nodes

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.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D130370