Fix for PR4235: to build a floating-point value from integer parts,

build an integer and cast that to a float.  This fixes a crash 
caused by trying to split an f32 into two f16's.

This changes the behavior in test/CodeGen/XCore/fneg.ll because that 
testcase now triggers a DAGCombine which converts the fneg into an integer
operation.  If someone is interested, it's probably possible to tweak 
the test to generate an actual fneg.

llvm-svn: 72162