[InstCombine] Avoid creating mis-sized dbg.values in commonCastTransforms()

[InstCombine] Avoid creating mis-sized dbg.values in commonCastTransforms()

This prevents InstCombine from creating mis-sized dbg.values when
replacing a sequence of casts with a simpler cast. For example, in:

  (fptrunc (floor (fpext X))) -> (floorf X)

We no longer emit dbg.value(X) (with a 32-bit float operand) to describe
(fpext X) (which is a 64-bit float).

This was diagnosed by the debugify check added in r335682.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335696 91177308-0d34-0410-b5e6-96231b3b80d8