[AMDGPU] Fix for vector element insertion

Summary:
Incorrect code was generated when lowering insertelement operations
for vectors with 8 or 16 bit elements.  The value being inserted was
not adjusted for the position of the element within the 32 bit word
and so only the low element within each 32 bit word could receive
the intended value.

Fixed by simply replicating the value to each element of a
congruent vector before the mask and or operation used to
update the intended element.

A number of affected LIT tests have been updated appropriately.

before the mask & or into the intended

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

Subscribers: llvm-commits, arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57588

llvm-svn: 352885