[committed, amdgcn] Zero-initialise masked load destinations

[Andrew Stubbs]

This is one of those things I don't know why we didn't notice sooner. 
The patch ensures that unused lanes in masked vector loads are 
zero-initialized, as per the internals manual.

This fixes an execution failure in testcase gfortran.dg/assumed_rank_1.f90.

When investigating the bug I got confused about the meaning of the 
"gather<mode>_exec" define_expand, which doesn't quite fit the pattern 
of the other "_exec" instructions. It's only used in one place so I've 
inlined it to avoid future confusion. It also reduces the likelihood of 
accidentally bypassing the zero-initialization in future.

I also needed a convenient way to create 0.0 vector constants without 
uglifying the machine description code, so extending gcn_vec_constant 
seemed like a useful place to do it.

Andrew
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