ix86: Improve __builtin_c[lt]z followed by extension [PR95535]

ix86: Improve __builtin_c[lt]z followed by extension [PR95535]

In January I've added patterns to optimize SImode -> DImode sign or zero
extension of __builtin_popcount, this patch does the same for
__builtin_c[lt]z.  Like most other instructions, the [tl]zcntl instructions
clear the upper 32 bits of the destination register and as the instructions
only result in values 0 to 32 inclusive, both sign and zero extensions
behave the same.

2020-06-05  Jakub Jelinek  <jakub@redhat.com>

	PR target/95535
	* config/i386/i386.md (*ctzsi2_zext, *clzsi2_lzcnt_zext): New
	define_insn_and_split patterns.
	(*ctzsi2_zext_falsedep, *clzsi2_lzcnt_zext_falsedep): New
	define_insn patterns.

	* gcc.target/i386/pr95535-1.c: New test.
	* gcc.target/i386/pr95535-2.c: New test.