[HLSL][SPIRV] 64bit firstbitlow returns incorrect values

[V-FEXrt]

Discovered while running the offload-test-suite. The values returned imply an incorrect lowering for the 64bit overload. This only occurs for the Vulkan target which makes sense as it has the most complex lowering.

# | Name:            ExpectedOut
# | Format:          UInt32
# | Stride:          16
# | Data:            [ 4294967295, 3, 63, 0, 4294967295, 3, 63, 0, 0, 3, 3, 
# |                    4294967295 ]
# | OutputProps:
# |   Height:          0
# |   Width:           0
# |   Depth:           0
# | ...
# | Got:
# | ---
# | Name:            Out
# | Format:          UInt32
# | Stride:          16
# | Data:            [ 31, 35, 31, 0, 31, 35, 31, 0, 0, 32, 31, 31 ]
# | OutputProps:
# |   Height:          0
# |   Width:           0
# |   Depth:           0

See llvm/offload-test-suite#209 for more context

[llvmbot]

@llvm/issue-subscribers-bug

Author: Ashley Coleman (V-FEXrt)

Discovered while running the offload-test-suite. The values returned imply an incorrect lowering for the 64bit overload. This only occurs for the Vulkan target which makes sense as it has the most complex lowering.

# | Name:            ExpectedOut
# | Format:          UInt32
# | Stride:          16
# | Data:            [ 4294967295, 3, 63, 0, 4294967295, 3, 63, 0, 0, 3, 3, 
# |                    4294967295 ]
# | OutputProps:
# |   Height:          0
# |   Width:           0
# |   Depth:           0
# | ...
# | Got:
# | ---
# | Name:            Out
# | Format:          UInt32
# | Stride:          16
# | Data:            [ 31, 35, 31, 0, 31, 35, 31, 0, 0, 32, 31, 31 ]
# | OutputProps:
# |   Height:          0
# |   Width:           0
# |   Depth:           0

See llvm/offload-test-suite#209 for more context