[DAGCombiner] Use the result value type in visitCONCAT_VECTORS

This triggers an assert when combining concat_vectors of a bitcast of
merge_values.

With asserts disabled, it fails to select:
fatal error: error in backend: Cannot select: 0x7ff19d000e90: i32 = any_extend 0x7ff19d000ae8
  0x7ff19d000ae8: f64,ch = CopyFromReg 0x7ff19d000c20:1, Register:f64 %1
    0x7ff19d000b50: f64 = Register %1
In function: d

Differential Revision: https://reviews.llvm.org/D55507

llvm-svn: 348759