implement an instcombine xform that canonicalizes casts outside of and-with-constant operations.

This fixes rdar://8808586 which observed that we used to compile:


union xy {
        struct x { _Bool b[15]; } x;
        __attribute__((packed))
        struct y {
                __attribute__((packed)) unsigned long b0to7;
                __attribute__((packed)) unsigned int b8to11;
                __attribute__((packed)) unsigned short b12to13;
                __attribute__((packed)) unsigned char b14;
        } y;
};

struct x
foo(union xy *xy)
{
        return xy->x;
}

into:

_foo:                                   ## @foo
	movq	(%rdi), %rax
	movabsq	$1095216660480, %rcx    ## imm = 0xFF00000000
	andq	%rax, %rcx
	movabsq	$-72057594037927936, %rdx ## imm = 0xFF00000000000000
	andq	%rax, %rdx
	movzbl	%al, %esi
	orq	%rdx, %rsi
	movq	%rax, %rdx
	andq	$65280, %rdx            ## imm = 0xFF00
	orq	%rsi, %rdx
	movq	%rax, %rsi
	andq	$16711680, %rsi         ## imm = 0xFF0000
	orq	%rdx, %rsi
	movl	%eax, %edx
	andl	$-16777216, %edx        ## imm = 0xFFFFFFFFFF000000
	orq	%rsi, %rdx
	orq	%rcx, %rdx
	movabsq	$280375465082880, %rcx  ## imm = 0xFF0000000000
	movq	%rax, %rsi
	andq	%rcx, %rsi
	orq	%rdx, %rsi
	movabsq	$71776119061217280, %r8 ## imm = 0xFF000000000000
	andq	%r8, %rax
	orq	%rsi, %rax
	movzwl	12(%rdi), %edx
	movzbl	14(%rdi), %esi
	shlq	$16, %rsi
	orl	%edx, %esi
	movq	%rsi, %r9
	shlq	$32, %r9
	movl	8(%rdi), %edx
	orq	%r9, %rdx
	andq	%rdx, %rcx
	movzbl	%sil, %esi
	shlq	$32, %rsi
	orq	%rcx, %rsi
	movl	%edx, %ecx
	andl	$-16777216, %ecx        ## imm = 0xFFFFFFFFFF000000
	orq	%rsi, %rcx
	movq	%rdx, %rsi
	andq	$16711680, %rsi         ## imm = 0xFF0000
	orq	%rcx, %rsi
	movq	%rdx, %rcx
	andq	$65280, %rcx            ## imm = 0xFF00
	orq	%rsi, %rcx
	movzbl	%dl, %esi
	orq	%rcx, %rsi
	andq	%r8, %rdx
	orq	%rsi, %rdx
	ret

We now compile this into:

_foo:                                   ## @foo
## BB#0:                                ## %entry
	movzwl	12(%rdi), %eax
	movzbl	14(%rdi), %ecx
	shlq	$16, %rcx
	orl	%eax, %ecx
	shlq	$32, %rcx
	movl	8(%rdi), %edx
	orq	%rcx, %rdx
	movq	(%rdi), %rax
	ret

A small improvement :-)



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123520 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstCombine/trunc.ll

@@ -97,3 +97,23 @@ define i64 @test8(i32 %A, i32 %B) {
 ; CHECK:   ret i64 %ins35
 }
 
+define i8 @test9(i32 %X) {
+  %Y = and i32 %X, 42
+  %Z = trunc i32 %Y to i8
+  ret i8 %Z
+; CHECK: @test9
+; CHECK: trunc
+; CHECK: and
+; CHECK: ret
+}
+
+; rdar://8808586
+define i8 @test10(i32 %X) {
+  %Y = trunc i32 %X to i8
+  %Z = and i8 %Y, 42
+  ret i8 %Z
+; CHECK: @test10
+; CHECK: trunc
+; CHECK: and
+; CHECK: ret
+}