[DAG] use getBitcast() to reduce code

[DAG] use getBitcast() to reduce code

Although this was intended to be NFC, the test case wiggle shows a change in
code scheduling/RA caused by a difference in the SDLoc() generation.

Depending on how you look at it, this is the (dis)advantage of exact checking
in regression tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@271526 91177308-0d34-0410-b5e6-96231b3b80d8