llvmpipe: convert double to long long instead of unsigned long long

llvmpipe: convert double to long long instead of unsigned long long

round(val*dscale) produces a double result, as val and dscale are double.
However, LLVMConstInt receives unsigned long long, so there is an
implicit conversion from double to unsigned long long.
This is an undefined behavior. Therefore, we need to first explicitly
convert the round result to long long, and then let the compiler handle
conversion from that to unsigned long long.

This bug manifests itself in POWER, where all IMM values of -1 are being
converted to 0 implicitly, causing a wrong LLVM IR output.

Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
CC: "10.6 11.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>