gallivm: Workaround http://llvm.org/PR18600

We have code generation paths that carry out swizzles of AoS vectors via
bitwise shifts, as these tend to generate more efficient code than
straightforward byte shuffles.  But when the input is a constant the
additional bitwise arithmetic operations somehow don't really get
constant propagated properly, evenutally causing assertion failure in
InstCombine pass.

Therefore avoid the bug by using the trivial shuffles for constant
inputs.

Although the sample LLVM IR can cause a crash with any LLVM version,
this was only seen in practice with LLVM 3.2.

Reviewed-by: Matthew McClure <[email protected]>
Reviewed-by: Roland Scheidegger <[email protected]>

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