Fix typographical and grammatical errors and inconsistencies.

The initial list of hits was generated with the codespell command
below, and each hit was evaluated and fixed manually while taking
context into consideration.

    DIRS="kernel/ frontends/ backends/ passes/ techlibs/"
    DIRS="${DIRS} libs/ezsat/ libs/subcircuit"
    codespell $DIRS -S *.o -L upto,iff,thru,synopsys,uint

More hits were found by looking through comments and strings manually.

techlibs/intel/cyclonev/cells_sim.v

@@ -54,7 +54,7 @@ module cyclonev_lcell_comb
    // Internal variables
    // Sub mask for fragmented LUTs
    wire [15:0] mask_a, mask_b, mask_c, mask_d;
-   // Independant output for fragmented LUTs
+   // Independent output for fragmented LUTs
    wire        output_0, output_1, output_2, output_3;
    // Extended mode uses mux to define the output
    wire        mux_0, mux_1;