[X86][SSE] Refactor getTargetConstantBitsFromNode to avoid large APInts (PR32037)

Much of PR32037's compile time regression is due to getTargetConstantBitsFromNode always creating large (>64bit) APInts during the bitcasting from the source data to the destination bitwidth.

This commit avoids this bitcast stage if the data is already the correct bitwidth.

llvm-svn: 305284