StgCmmPrim: remove an unnecessary instruction in doNewArrayOp
Previously we would generate a local variable pointing after the array
header and use it to initialize the array elements. But we already use
stores with offset, so it's easy to just add the header to those offsets
during compilation and avoid generating the local variable (which would
become a LEA instruction when using native codegen; LLVM already
optimizes it away).
Signed-off-by: Michal Terepeta <email@example.com>